Although molecular biologists know something of the interactions involved, they do not know the whole story. There will be unpredicted and unpredictable biochemical outcomes in the life of the plant. These will be toxic in very few cases or to a small proportion of susceptible people. But the risk is real.
The statutory testing of genetically modified foods involves measuring levels of known toxins and allergens similar to those listed by Dawkins. It is the unpredicted toxins arising from the GM process that are not tested for. This is what the fuss is about and it is disingenuous of Professor Dawkins to imply a superstitious, ignorant basis to our concerns.