You could make a similar argument about any technology: we don't fully understand nature, so messing with it is dangerous. Luckily we've come up with techniques of science and engineering that help us mitigate the danger. We can run experiments, do focused testing, build and validate models, and so on. Is there something unique about GMOs that would make these techniques not work? How do you feel about pharmaceuticals that often "play god" at a more fundamental biochemical level with even more poorly-understood mechanisms?