what you have heard is speculation - and not too well founded
I've even heard people try to claim that the Uwaves excite molecules with a bias to translational vs. vibrational motion - you can go look those up if not clear from your chemistry courses.
A good place to look would be Cons. Reports - it would be in old issues. No one seriously questins this now
AFAIK.
uwave cooking is less dangerous than BBQ or any fry technique that creates black or brown colored parts of the food or sauce. These yummy cooking techniques DO create nitrosamines, polycyclic Aromatic Hydrocarbons (PAHs) and other things that are NOT good for you - carcinogenic, toxic etc.
As Bruce Ames has pointed out repeatedly however, the body has a series of mechanisms to deal with these toxins. Of course, just cuz you can recover from something does not mean you chould challenge your body with it over and over again.
Generally speaking, you should cut the BBQ down or out if you are a child, are pregnant or working on it. Otherwise, I wouldn't worry too much but would avoid too much black on the food.
I wouldn't worry about the uwave effects on food AT ALL.