Genetic modification starts with a very small gene pool for a species. Recall that all subsequent generations come from only a very small number of individuals used in the genetic modification.
Like a clone, there is a very high level of consistency between individuals, so much so that any disease affecting any plant in the whole world that came from that genetic modification is immediately highly susceptible to spread of that disease.
Just as the Irish Potato Famine occurred because the main crop was vegetative reproduced , and so had very few ancestors with enormous numbers of offspring, like our GM plants, and both early blight and late blight destroyed most of the crop country wide, If a country becomes dependent on crops that are severely limited in this way, a disease that wipes out most of that crop is highly likely.
Genetic modification in theory could offer us some wonderful crop that produces enormous crops like corn but has the nutritional value of millet, rice, soy, and butternut squash al in the same crop. Now who on earth would not want to grow that?
So we all grow it, and become dependent on it as our dietary staple, and a disease develops that can wipe out the whole crop. All the benefits we hoped to gain are erased. We might have stored some crop to tide us over for a year or two, but what then?