There are two main problems with the genetically modified plants especially:
1st = The specific allergies that children develop to certain plants such as peanuts. If these types of plants are modified by scientists then new allergens can be introduced to the plants thus causing unexpected results from various people.
2nd = There are many unknown effects that come with GM foods because many of the tests for these have only been performed on small rodents such as rats. When performed on these creatures, they are seeing a poor condition of the digestive tract when GM food is eaten.