There are legitimate concerns... Low genetic diversity is already a big problem, especially when an artificial species outcompetes indigenous variants only to all die to the same random illness.

GMOs aren't inherently different but make it significantly easier and cheaper to fuck up ecosystems in this way, which is the most convincing argument I've heard against them to date.

Also the patenting of crops is a terrible practice that needs to die, and GMOs are just amplifying the problem by making "patent-free" crops even less competitive.

Finally we already have more than enough food in the world to feed everyone (we just lack the ability/willingness to properly distribute it), and that's before taking into account just how much of farm land is used for cattle we don't actually need or to grow cattle feed (it's a LOT). So in this way GMOs kinda sound like a solution looking for a problem (at least when only viewing them as yield/profit multipliers). Doesn't Monsanto make enough money already?