Should I judge the efficacy of Samadhi based on the ethical qualities of the meditator?

The Buddha told Ananda to not judge others, because Ananda did not have the spiritual development to be able to judge others. Instead, you should judge your own moral & spiritual development. Are you still a puthujjana? Are you still obsessed with sex or immorally view women as sex objects? Are you ungrateful towards those who teach you Dhamma? Some suttas:

50. Let none find fault with others; let none see the omissions and commissions of others. But let one see one's own acts, done and undone. Dhammapada

Don’t Judge Others!

Therefore, Ānanda, you should not be a hasty critic of people, should not lightly pass judgement on people. One who passes judgement on people harms himself. I alone, Ānanda, or one like me, can judge people.

AN 6.44

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68. How to Judge a Person’s Character

Four facts about a person, O monks, can be known from four circumstances. What are these four?

By living together with a person his virtue can be known, and this too only after a long time, not casually; by close attention, not without attention; by one who is wise, not by one who is stupid.

By having dealings with a person his integrity can be known, and this too only after a long time, not casually; by close attention, not without attention; by one who is wise, not by one who is stupid.

In misfortune a person’s fortitude can be known, and this too only after a long time, not casually; by close attention, not without attention; by one who is wise, not by one who is stupid.

By conversation a person’s wisdom can be known, and this too only after a long time, not casually; by close attention, not without attention; by one who is wise, not by one who is stupid.

AN 4.192

Since it is impossible for an unenlightened being to know the quality of another’s mind (that being a siddhi that comes only with enlightenment) there is no way to judge the quality or character of someone else’s practice.

However, we can see the character of an individual in their way of being towards others, which may call into question the true extent of their accomplishment—are they rude? boastful? Do they retaliate in anger when questioned? Do they lack compassion towards others? Etc.

Thus, a clue for us can be found in this teaching of the Buddha:

“The reason for practicing dhyana and seeking to attain Samadhi is to escape from the suffering of life, but in seeking to escape from suffering ourselves, why should we inflict it upon others? Unless you can so control your minds that even the thought of brutal unkindness and killing is abhorrent, you will never be able to escape from the bondage of the world's life. No matter how keen you may be mentally, no matter how much you may be able to practice dhyana, no matter to how high a degree of Samadhi you may attain, unless you have wholly annihilated all tendency to unkindness toward others, you will ultimately fall into the realms of existence where the evil ghosts dwell.”

This quote is part of a teaching on the need to follow various precepts. Obviously, if one must follow precepts for one’s behavior in order to reach enlightenment, not doing so is a clear indicator that one’s practice is not correct. So I would say, in answer to your question: Yes, you can judge the efficacy of samadhi by the ethical qualities of the meditator—but you should only do this as part of an evaluation of a teacher, and not judge others indiscriminately because it can lead to feelings of superiority in yourself, and cause the one being judged to become doubtful, and perhaps lose their way.

Until we ourselves are fully enlightened, we are susceptible to slips and mistakes, along our path.

The above quote is taken from the “Surangama Sutra,” Chapter Two, “Importance of Keeping the Precepts.”

Right view, good householder, makes the different whether one is an insider, or still an ordinary outsider:

"Lord, the disciple of the noble ones in whom the factors of stream entry are altogether & in every way lacking: Is he called a disciple of the noble ones who lives heedlessly?"... Nandiya Sutta: To Nandiya

And it's also clear that one, even if a talented meditator, as very popular in the west, modern world, if his virtue is wrong, the whole path is wrong.

So is it that from wrong view comes wrong resolve, wrong virtue... wrong effort, wrong mindfulness, wrong samadhi... wrong release.

But one should be clear, that as long not insider, hardly ever know who is and who isn't. So ones right effort primarly is to straighten ones right view, resolve, action, speech and livelihood, and not to meditate... since right Samadhi, right wisdom and right release comes by having given the right causes by those causes.

Although western, modern, wrong introduced, think that meditation is the importand pratice, actually training in virtue and dhamma learning, hearing, discussion, are foremost to master and right practice for one not having reached what can be reached

[Note that this isn't given for stacks, exchange, other word-binding trades but for an escape from this wheel]

But, how important and relevant is the behavior of the meditator (towards others and in general) as a factor to check whether his/her meditation style is right or wrong?

It probably says more about their sila habits. This is of course related to other practices (like panna or samadhi) as well.

Regarding your question, we ultimately need to be "a light unto ourselves" regardless of what other says.

https://www.accesstoinsight.org/tipitaka/sn/sn22/sn22.043.wlsh.html

Also, as an ideal maintaining equanimity towards others is possibly more beneficial as an alternative to figuring out ways to evaluate them. Even though it may seem odd or even wrong from an observers perspective, we are all trying our best to get by in life.

You can judge the efficacy of the samadhi (and the rest of his practice, including ethics/ virtues/ sila) based on how the meditator reacts to things that he does, and to things that happen to him.

If someone scolds him and this makes him wallow in depression, then he has not progressed on the noble path compared to one who gets over it quickly.

If he does something wrong and this makes him wallow in remorse and depression, then he has not progressed on the noble path compared to one who gets over it quickly.

Also, one who is progressed on the noble path would have better control over the three poisons. This should indirectly result in better ethical conduct. So, yes, ethical conduct may indirectly provide information on the efficacy of samadhi.

Additionally, anyone who intends to progress on the noble path is expected to stick to the five precepts as closely as possible. Perfection in the practice of the five precepts is only expected of Arahants. For unenlightened persons, persistent attempt at heedfulness (appamada) in the practice of the five precepts is sufficient.

From AN 3.100:

Take the case of a person who does a trivial bad deed, but it lands them in hell. Meanwhile, another person does the same trivial bad deed, but experiences it in the present life, without even a bit left over, not to speak of a lot.

What kind of person does a trivial bad deed, but it lands them in hell? A person who hasn’t developed their physical endurance, ethics, mind, or wisdom. They’re small-minded and mean-spirited, living in suffering. That kind of person does a trivial bad deed, but it lands them in hell.

What kind of person does the same trivial bad deed, but experiences it in the present life, without even a bit left over, not to speak of a lot? A person who has developed their physical endurance, ethics, mind, and wisdom. They’re not small-minded, but are big-hearted, living without limits. That kind of person does the same trivial bad deed, but experiences it in the present life, without even a bit left over, not to speak of a lot.

Suppose a person was to drop a lump of salt into a small bowl of water. What do you think, mendicants? Would that small bowl of water become salty and undrinkable?”

“Yes, sir. Why is that? Because there is only a little water in the bowl.”

“Suppose a person was to drop a lump of salt into the Ganges river. What do you think, mendicants? Would the Ganges river become salty and undrinkable?”

“No, sir. Why is that? Because the Ganges river is a vast mass of water.”

“This is how it is in the case of a person who does a trivial bad deed, but it lands them in hell. Meanwhile, another person does the same trivial bad deed, but experiences it in the present life, without even a bit left over, not to speak of a lot. …

Should I judge the efficacy of Samadhi based on the ethical qualities of the meditator?

No, you cannot do this. Jhana only temporarily suspends the 5 Hindrances / defilments. So when it is suspended the meditator may act more ethically than otherwise. Also, when not in Jhana sometimes hindrances may arise them he might be less ethical than when the Hindrances are suspended.

As far as I understand, one of the main purposes of the eight factor of the Path (samma samadhi) is to clean the mind from impurities.

Samadhi temporarily suspends the defilements so you can develop wisdom which suspends it completely.

I've read a lot of discussions where one meditator states that only his/her method is the true method, while all the other methods are worthless and wrong. There are multiple arguments for such beliefs, and one might agree with them in some degree.

If a certain technique works for a person he might believe this is the only method that works through there might be other valid techniques.

Does it follow the instructions found in the suttas? Does it interpret correctly those instructions? Does it lead to a temporary peace of mind? Does it stops during the extention of the sitting the arising of the hindrances? Can the steps followed be replicated with consistency? Can those steps be followed by everyone else? Does it lead to the arising of the factors of jhana?

All these are valid ways to validate the technique.

But, how important and relevant is the behavior of the meditator (towards others and in general) as a factor to check whether his/her meditation style is right or wrong?

When meditating a person may relatively become a better person. But one has to become enlightened to become a perfect person. Just observing another you cannot easily judge if the technique is right or wrong.

Also, you do not know if the meditator is practising the technique right in the first place.

Best is one does the practice and if it works for oneself one know this works. If not then this does not mean it is invalid but it could be it does not work for oneself.

If a reknown meditator (even openly self acknowledge) shows little to no empathy, prudence or wisdom, should I suspect that his/her meditation method is wrong?

Some meditators may show quick results while others may get slow results. So it is difficult to judge the technique by looking at one individual and a few individuals. If you have a reasonably large sample of meditators then you might be able to make this inference.

Or to maybe put it other words: is an ad-hominem argument valid in these scenario?

The appeal of a single or few people cannot bee representative of the validity of any technique. You need to look if a reasonably large sample of people practices technique X vs Y what actually give more significant results.

Right immersion (sammāsamādhi) requires right view. Without that right view, we step off the Noble Eightfold Path very early on (AN4.208):

And who’s bad? It’s someone who has wrong view, wrong thought, wrong speech, wrong action, wrong livelihood, wrong effort, wrong mindfulness, wrong immersion, wrong knowledge, and wrong freedom. This is called bad.

But there are also many right ways to practice the Noble Eightfold Path, and one may find many teachers for each of those ways. Indeed, there are many suttas that discuss different approaches:

“Reverend Moggallāna, there are four ways of practice. What four? Painful practice with slow insight, painful practice with swift insight, pleasant practice with slow insight, and pleasant practice with swift insight. These are the four ways of practice.

In sutta AN4.167, Moggallāna tells Sariputta that:

I relied on the painful practice with swift insight to free my mind from defilements by not grasping.

Yet in that very diversity of teaching, there is always a shared right view. No killing, no lying, no stealing, etc.

Choose a teacher with care. I would not learn honesty from a thief, nor would I follow any famous teacher who punches students. Ethics (sīla) is quite important.