What Viviane said, at least from a health perspective.
However, also because of evolution, any direct modification to the genetic makeup of an organism, could change evolutionary patterns that have long term effects on the ecosystem overall. Not that this is by necessity an issue, just part of what happens with evolution. Sometimes good, sometimes bad, sometimes no known immediate effect.
Think Africanized bees. When African honeybees were introduced to Europe and North America in the 1950s to improve honey production, it was not expected these bees would take over European bee populations, and produce so-called "killer" bees. By the 1970s, these relatively aggressive bees were overpowering European bee populations, and replacing them, earning the name "killer". At that time it was thought these would potentially become a serious threat to humans. That threat was unsubstantiated.
Longer term, beekeepers have adapted, and found ways to keep these bees in their farms, and use them for productive honey producing purposes. Likely, any evolutionary change forced by GMO will force some adaptation in the future, at the worst.