Are microwave ovens harmful to human health?
Define "Dangerous to our health." I wouldn't put anything alive in one. Yes, that would be dangerous to that alive thing's health. If you carried a microwave oven around, you could drop it on your foot.
If you put the wrong materials or food (like sliced grapes, go figure!) you could set the thing on fire. That would be very dangerous. Raw eggs in their shells can explode if heated in a microwave as can food in any sort of sealed container.
Any appliance that heats things should be treated with caution. It is very easy to over-heat food in a microwave and to burn ones self, just like with food cooked on a stove or in a conventional oven.
One must be especially careful heating liquids in smooth-sided containers in the microwave as they can super-heat without boiling. The smooth sides prevent bubbles from forming but even slightly agitating the container (like while taking it out of the microwave) could cause it to suddenly bubble over and could cause burns. Always put a wooden chop stick in any liquid being heated for more than a few seconds or so.
If you are asking whether eating food cooked in a microwave oven, absent any burns or kitchen mishaps is somehow inherently more dangerous than other cooking methods in that it makes the food "radioactive" then no. Microwave cooking is no more disruptive or dangerous to the wholesomeness, nutrition or safety of the food than is conventional cooking. Heat is heat.
For those who say that it breaks down enzymes, well cooking, chewing, eating and digesting the food also breaks down the enzymes in the food. That's why we have enzymes in our own digestive tracts. We don't use the enzymes in the food for nutrition or digestion. For those who claim that it breaks down nutrients, some nutrients are destroyed by cooking, other nutrients are made more available to the body by cooking, irrespective of the way the food is cooked.
To paraphrase Gandalf, "Dangerous! And so am I, very dangerous...and Aragorn is dangerous...You are beset by dangers, Gimli son of Gloin; for you are dangerous, yourself."
Update: Oh, and they cannot in any way, shape or form, protect people from their own stupidity: YouTuber Almost Dies After Cementing His Head Inside A Microwave