With regards to increased proliferation or better adaptation to more climates, the fear that they will somehow destroy the ecosystem is quite unfounded - if a plant which was slightly better adapted could do this evolution would have assured mutual destruction by know already. Remember that modern varieties of crop plants are probably not well suited to growth outside of human cultivation, being bread to human needs which may be maladaptive in the wild. GMOs are no more capable of taking over the world than their wild type counterparts I assure you - we are years and years away from being able to design something so effectively!

What worries ecologists is not out of control proliferation of genetically modified crops, but unpredictable hybridization with natural species. Plants hybridize considerably more readily than animals, and an agricultural concentration of a particular plant could introduce a large genetic load to the local population if it's wind or insect pollinated. Although usually not dangerous, the occasional propensity of hybrid species to fuck ecologies up in ways that neither of their parent species do is worth being wary of.