The basic of this theory are rather flawed.
Living things have had millions of years to engage in a evolutionary massive arms race. Defense has kept up with offense. Evolution is all about using random processes. Evolution has already given us the full set of defenses we need to change from 'random' or 'unintentional' attacks based on genetics. That's why we have immune systems with white blood cells, variant blood types, skin, mucus, fevers, blood-brain barriers, etc. etc. etc. etc.
The basic belief that human caused mutations will randomly create something dangerous demonstrates tremendous ignorance of evolution. It's like they believe in creationism.
I am not saying we can't get around these defenses. We can. But not by accident. The only truly harmful species will have to be intentionally designed by humans that go out of their way to make a dangerous life form, i.e. a plague genetically engineered to kill people.
But for every single 1 intentionally designed genetic species, there are (and will always be) millions of random mutations from cosmic rays, sunlight, etc. As the humans are not trying to make the gene engineered species dangerous, the chance of it happening are FAR more likely in the natural mutations than in the genetically created mutations.
Throw in the extensive testing that humans do to their genetically engineered species (that does not occur in the wild mutations), and you get a guarantee that for every single human engineered life that gets a dangerous trait by random chance, there will be 10 (or more) randomly evolved life forms with mutations we call dangerous.
Now, we might get things that inconvenience us - food that tastes bad or turns a funky color, etc. etc. Even something like a slightly greater cancer risk is just an inconvenience, not a real problem. We already risk that with non-gene engineered stuff. Basically, I am saying that a genetically engineered sugar substitute will be no more risky than Saccharine - which is still legal.