Are there any countries where modern devices that take 100-240V AC power won't work?

“Universal” supplies are just that

Utility power available to general consumers and travellers is going to be in the 85-265VAC/47-63Hz range that your average “universal input” SMPS is rated for. Reputable (read: listed) supplies are tested to every corner of this range (although some are only rated to 90VAC min).

Not having a grid isn’t a problem either

Modified square waves (+peak, 0, -peak, 0, +peak) shouldn’t be an issue — the only potential problem is with active Power Factor Correction circuits misbehaving in mild ways (such as not providing proper PFC, or emitting undesired audible noises). (Keep in mind that just about all consumer standby power supplies and many low-end inverters generate MSW output.) Likewise, wandering frequency (from a generator) won’t harm the supply much if at all — the PFC circuit generally has enough bandwidth to track a source that drifts.

Noise filtering is a necessity on SMPSes anyway, so they generally reject external noise. Spikes are sometimes suppressed by a metal-oxide varistor or MOV (same thing you’d find in a surge strip), but are probably the greatest hazard unstable power can pose to a modern SMPS.

Weird and wild power sources

DC, interestingly enough, provided the voltage is reasonable (locomotive auxiliary ~70VDC is pushing it, but say a 115VDC or even a 380VDC rail should be OK), isn’t an issue. In this case, the PFC circuit, if present, simply acts as a straight boost converter, and the input diodes do (almost) nothing.

Likewise, 400Hz aircraft/marine power sources won’t cause much trouble either, although supplies listed for such use are generally only seen on more specialized gear, such as test equipment. Most input diodes can deal with it, and most PFC circuits have the bandwidth to roughly follow a 400Hz sine, albeit with degraded performance.

This leaves one more oddball to cover, and it hails not from some out of the way place, but North American industry. It’s the 277V/60Hz single phase power sometimes found there, and it probably comes the closest you’ll ever see to thwarting the universal-ness of your supply — a reputable universal supply will start up and run on it, but with reduced margin for surges as the peak DC bus voltage is around 390V vs 340V for 240V mains in. Thankfully, 277V is rare to see at a receptacle even in large buildings in the US, and you wouldn’t be able to plug a normal North American NEMA 5 plug into it anyway! (277V single phase uses the NEMA 7 receptacle.)

The most unusual power supply that I have encountered is the Philippines: 220V (or maybe 230V, I forget) 60Hz. So, a European / most of the world style voltage but a North American style frequency. This used to cause an issue with some of the larger computers and the hardware engineers would complain about it. However, most modern consumer electronic devices are quite happy there.

Further trouble is caused by their use of North American style sockets. So, US devices will fit and appear to be suitable but will often go bang when used. This will still be probable for many simpler devices e.g. light bulbs, hair driers, etc. Most electronic gadgets now accept a wide range of supplies but don’t assume that all devices do.

If you are going to quite out of the way places then you may want to consider the quality of the supply. The nominal voltage may be suitable but there could be nasty spikes. You might want to research protection against that.

According to Wikipedia (in New York City): In January 1998, Consolidated Edison started to eliminate DC service. At that time there were 4,600 DC customers. By 2006, there were only 60 customers using DC service, and on November 14, 2007, the last direct-current distribution by Con Edison was shut down.

That means the last place that used DC current discribution was definitely eliminated in 2007. Never mind voltage and frequency differences, plugging your average power adapter into DC could be catastrophic (especially if there is a transformer involved). But I guess there were no “publicly” accessible sockets years before that.

Note that there are still special purpose DC sockets used from place to place (mostly 48V), but fortunately all of them should be incompatible with the “common” A/C mains plugs.

All “normal” mains power supplies should be OK. Most ‘universal’ supplies will work down to 90 VAC. Most switch mode supplies convert the AC to DC and then deal with that.

You can find exotic systems – but not in normal use. Maybe shipboard or aircraft in extreme cases – but nothing that they would supply to members of the public.

Rarely in “out of the way” places you may encounter non-standard supplies that could cause problems – usually at remote or isolated locations. Usually where connection to a national grid is not available and power is produced locally. Examples might be diesel or LPG (usually) powered alternators, and low voltage to mains supplies using electronic inverters.

Even in such situations larger alternator systems usually produce properly regulated voltages. Frequency may wander somewhat during large loads or load variations. Voltages may be low under heavy loading.

“Bottom end” electronic inverters may produce quasi-sine-wave outputs – rather than providing a sine wave the output goes 0, +V, 0, -V, 0 … with the on to off ratio adjusted to provide an approximation to mains AC for most equipment. Some power supplied (laptops, some PCs, others) may be damaged by applying these step waveforms due to input filter capacitors. (I have heard power supplies which buzz loudly in such cases. Some few systems may see these as too high or too low in voltage.)

According to Wikipedia, the minimal voltage is 100 V in Japan and the maximum voltage 240 V in several countries. As for the frequency, they all lie between 50-60 Hz. So there isn’t any country with such an exotic voltage or frequency that wouldn’t fit the range you mentioned.

If you think of it, it wouldn’t make sense for a small country to go too far outside the two common voltages, as that would mean that you’d have to produce a range of electric devices just for that one country.