Biotechnology through Genetic engineering has made food crops more resistant to disease, but the mere act of modification of the naturally selected food crops may actually disturb the delicate balance of biodiversity which exists in nature causing a disturbance to the natural balance.
The production of GMOs has negative impacts on the natural ecosystem which are not apparent now but will be apparent in the future. For example, genetic changes in a particular plant or animal might render it harmful to another organism higher up in the food chain and ultimately this effect may build up to destroy the entire food chain in which that plant plays a role.
GMOs have been known to retain some of the genetically modified DNA in the final product made for human consumption. Such remnants of genetic material are harful to human health and can cause production of previously unknown allergens.
Genetically modified plants and animals have the potential to replace traditional farming or say poultry and meat-producing practices. This will result in destruction of economies based on these products.
In the context of applications of genetic engineering in human life, misuse of this technology in the production of biological warfare or weapons is a very major disadvantage.
Genetic engineering is being used to create human organs but in the long run if it can create genetically modified, perfect human specimens who are better than the creators than this may be disastrous.
Nature selection in man and the resulting diversity of the human genetic pool is essential for the survival of the species. Genetic engineering will interfere with this process too causing unknown complications.