Genetic engineering is not the same as cross breeding which has been practised for many years. It is not an exact science. For example, it could result in some dangerous organisms becoming resistant to antibiotics. It could accidentally create new poisons and diseases. It could result in weeds and insects becoming resistant to pesticides and herbicides.
Food comes from nature.
If we change the fundamental make-up of a food, it could cause disease, just as herbicides and pesticides were found to in the past. Genetically modified foods do not have to be tested before they are sold in the shops. Plants designed to be herbicide-resistant will allow higher concentrations to be sprayed on the crops, so that food contains more chemicals, and rivers and reservoirs become more polluted. Genetic engineering of livestock leads to sick and suffering animals and to worse quality food. Already, animals are being bred with diseases for experiments and a life of suffering.