There are a lot of factors. One is speed. If a plant has been naturalized for centuries, and the selective breeding changes are (by nature) small, then other organisms have time to adapt and humans have time to consider consequences. When the changes are as fast as GMO makes possible, then you can have a radical change with unintended consequences and no real time to think about them before the changes are in effect -- as in the case of plants made immune to certain poisons. I'm not sure whether this was ultimately determined to be the case, but I remember not long ago people were talking about the collapse of the mid-continent Monarch butterfly population because of the overnight and widespread use of a specific insecticide to which plants had been made immune.
But those things are not the only things that GMO does. GMO also makes possible more productive plants even without those specific negatives. New wheat and rice varieties probably helped reduce famine in Indian, parts of Africa and parts of China, simply be being more productive. If you look at a painting called "The Harvesters" by Pieter Bruegel, probably painted in 1565, you can see people harvesting a wheat crop in which the wheat is literally as tall as the people. That's an accurate rendition, and meant that the wheat grains themselves were a small fraction of the actual plant, and the rest of the plant, straw, had few uses other than things like horse bedding. Our current wheat is barely knee high, and much more efficient in all ways. That's fairly rapid selective breeding, and I don't see the harm however that is done.
The same guy would convince you that people then were uniformly pig ugly, deformed and totally stunted in their growth, making the accuracy of the relativity ratio between stalk height and persons even more suspect.
It's the same kind of presumption you could easily make from looking at Russ Meyer snaps from the 50s/60s: all American women have massive tits, tiny waists, generous asses and an overwhelming desire for men of all sorts - as long as they buy the magazines. Other, more recent publications, indicate that the "woman on the street" may more readily resemble the barrage balloon. Where lieth the truth?
Perhaps it took painters longer to get to grips with perspective than is generally thought...
;-)
P.S. Congratulations on the double best-sellers listing!
https://theonlinephotographer.typepad.com/the_online_photographer/2018/10/number-one-ot.html
