Re: GM Crops can cause problems themselves.
It's not quite the same thing because the insertion of the genetic material is done using a viral injection system. There's also the issue of the marker genes you insert alongside your new gene which indicate that there's an increased chance that you've hit the mark - things like making certain sugars, glowing in the dark, that kind of thing. Plus there's the chance that you will disrupt a control sequence which might turn on a dormant gene that makes a poison or a potent allergen, or that you will turn OFF a gene that makes some nutrient. Besides, the leakage of genetic material is the LEAST worry of the crowd ringing the alarm bell. It's what you might call the indirect effects of the implementation of the technology.
Such as increased use of herbicides and pesticides, particularly those partnered to a product like Roundup which can lead to super weeds and a loss of biodiversity in the field and its margins, the subsequent loss of or damage to predator species, such as the DDT that built up in the eagles that ate the lizards that ate the bugs and the mice that ate the crops that the DDT was sprayed on.
Selective breeding involves crossing species, selecting those with characteristics you desire and refining those breeds. It takes years, and at every step you are checking. Even so, mistakes arise using that kind of husbandry too.