Dangerous for the environment
Genetically manipulated organisms could cross with other organisms.
This could cause unpredictable and irreversible changes to the ecosystem.
Nobody knows the effects on the long term
Nature is an extremely complex inter-related chain consisting of many species linked in the food chain. Some scientists believe that introducing genetically modified genes may have an irreversible effect with consequences yet unknown. Genetic manipulation is still in its infancy and nobody knows from practical experiences what the consequences could be.