There are different types of dehumidifiers, typically you'd use a refrigerant based system. Without proper ventilation the compressor can overheat, the refrigerant is also typically flammable, lubricants also. Design flaws and not maintaining the system can cause reliability issues. If a device has been designed by a competent team, there should be a thermal cutoff ie when the motor gets too hot it will shut down. A badly designed system may not have this at all, or the thermal sensor might be somewhere stupid meaning the measured temperature isn't the one that matters, its also possible the cut off is too high. It's also possible a crap compressor/motor is being used and driven too hard for the use case.
Bad wiring is also possiblly an issue.
With a peltier system. They use a lot of power. Components can get really hot such as the power electronics used to drive the peltier cooler.
Another poster mentioned water and electronics together with poor design can cause fire risks. This is absolutely true.
Without actually having these failed devices I can't really give much more insight than that. But I hope it gave anyone a basic idea.