The concern with using them for crops is not so much that they are dangerous to eat (that hasn't been conclusively established one way or the other yet) but that there are some other unintended consequences. For example, there is a kind of GMO corn called "Roundup-Ready." It has been developed to be resistant to glyphosate (Roundup) which kills any kind of plant. Glyphosate is less toxic than some other herbicides, so if you are going to use any herbicide at all, it's better than the others. But there's no way to apply it to a field to kill the weeds without also killing your crop, which is why they developed "Roundup-Ready" seeds. Then glyphosate can be used to kill the weeds without endangering the crop. However, nature always bats last, and because of the widespread use of glyphosate some weeds have managed to become resistant to it. The worst of these is called Palmer amaranth, which is a very nasty weed that is almost unkillable, and it's begun infesting a lot of fields in the south and moving north. It chokes out everything. The result is that even if you have planted GMO corn that's resistant to glyphosate, the glyphosate-resistant weeds will kill your crop anyhow. It's Mutual Assured Destruction with plants, again proving the point that it's not nice to fool Mother Nature.
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