And the front (cornea) has more pain receptors per surface area than any other part of your body. Not that this would necessarily help prevent cataracts if you expose your eyes to a magnetron.
I didn't realise this until I had laser surgery to correct a cornea problem. Oh dear, two days in a dark room on morfin, before I could even think straight again, no fun. But now I can see better again.
It's also a problem that there is no blood flow through your lenses. If you heat them via microwave radiation, they're going to stay hot for an alarmingly long time.