[pg-gadfly]

Behavior of microwaves is well-understood yes. Proving a negative would be rather difficult however, much like I find it hard to prove that no magical gnome exists at the bottom of my well, or that there is no renaissance-era teapot floating at the other side of our solar system perpendicular to our planet.

[im3w1l-alt]

What we all want to hear is that they blasted some poor animals with radiation their whole lives and nothing happened. And that they exposed people for a few years to no ill effects.

You know kinda like they do for new medicines.

[pg-gadfly]

Observing complex reactions with chemicals requires much longer study than simply beaming things with microwaves. Results are nearly immediate, and any long term effect would have to be shared with everything else that warms things.