VBD
Microwaves are part of the electromagnetic spectrum, and lie between radio and infrared. They are not ionizing radiation like x-rays and emissions from radioactive decay. They have no carcinogenic effects. They heat similarly to infrared, only the heat penetrates deeper due to the longer wavelength.
In general, cooking with microwaves changes food less than many other conventional cooking methods. Infrared radiation (grilling or broiling) deposits all the heat energy on the surface, and thus changes food FAR more than a similarly cooked food in a microwave. Just take a look at a piece of broiled meat vs a microwaved one… Also compare a pressure cooked pork shoulder… Or a smoked salmon.
The longer cook times and high external heat of conventional cookers causes charring, tissue breakdown, and caramelization. All those effects come with significant chemical changes to the food.
The rapid cook time and more even penetration of the microwave usually means there is less chemical change to the food, not more. (note that conventional rapid-cook methods such as steaming and stir-fry will be similar)
Now as for taste… those crispy little chemical bundles you get from the oven or fry pan do taste better than the relatively unchanged microwave food. So there IS a difference.