Is reiki harmful or not (is it harmful to advancing in the path)?

Basically, Reiki is healing by means of feeling energy and letting it to harmonize. It might help to be healthier, to develop inner and outer harmony, and therefore to advance in Buddhist practice more easily.

For example, it can happen that if our feelings are coarse, we remain tense, and our practice of calming does not go well. But if our feelings are subtler, the harmony is deeper and calming practice progresses better.

In Tantric Buddhism, energy practices are a part of the training. In Zen they are used for good health and swift progress.

Therefore I think that Reiki is not harmful in itself. It might become a distraction, but also it might become somehow helpful on Buddhist path.

Reiki harmeful or not ? is it harmeful to advancing in the path?

In the famous Maha-parinibbana Sutta, there is a passage where the Buddha explains to the wandering ascetic Subhadda, that "in any doctrine & discipline", that does not contain the Noble Eightfold Path, enlightened beings cannot be found.

That is very important since it shows that only teachings that contain the noble eightfold path can lead to freedom from suffering, to Nibbana.

The quoted passage can be found below.

Then Subhadda went to the Blessed One and exchanged courteous greetings with him. After an exchange of friendly greetings & courtesies, he sat to one side. As he was sitting there, he said to the Blessed One, "Venerable sir, these brahmans & contemplatives, each with his group, each with his community, each the teacher of his group, an honored leader, well-regarded by people at large — i.e., Purana Kassapa, Makkhali Gosala, Ajita Kesakambalin, Pakudha Kaccayana, Sañjaya Belatthaputta, & the Nigantha Nataputta: Do they all have direct knowledge as they themselves claim, or do they all not have direct knowledge, or do some of them have direct knowledge and some of them not?"

"Enough, Subhadda. Put this question aside. I will teach you the Dhamma. Listen, and pay close attention. I will speak."

"Yes, lord," Subhadda answered, and the Blessed One said, "In any doctrine & discipline where the noble eightfold path is not found, no contemplative of the first... second... third... fourth order [stream-winner, once-returner, non-returner, or arahant] is found. But in any doctrine & discipline where the noble eightfold path is found, contemplatives of the first... second... third... fourth order are found. The noble eightfold path is found in this doctrine & discipline, and right here there are contemplatives of the first... second... third... fourth order. Other teachings are empty of knowledgeable contemplatives. And if the monks dwell rightly, this world will not be empty of arahants."

I'm not sure that it's effective (see e.g. Is Reiki scientifically valid/proven?). So although it may be harmless (e.g. having no effect) it may be a distraction from any other treatment or activity (or non-activity) that is effective ... and it that sense (i.e. as a distraction from what's effective) I'd guess it may be harmful.

It's definitely a distraction. There's something be said for the whole-hearted practice of a single path. The more we pursue, the more scattered our attention, and inevitably the less directed our efforts. I would also have to ask after your motivations. Are they to heal or are they to simply to become someone who can heal? Is what's driving you down that road curiosity, compassion, or perhaps something more sinister and insidious? Lastly, just something is true for reiki practice doesn't necessarily mean it would transfer well into Buddhist practice. Generally, the more ideas we have in our heads about what practice is, what it should look like, and how it should feel the more likely we are to run into dead-ends and obstacles.