Well, you wouldn't want to modify a gene that isn't well-characterized, sure. But there are some things that are known with a great deal of certainty.
The issue I have with this type of engineering is there is no means of egress (if you will), no thought about how a recall would be implemented.
Humans are flawed. The history of engineering displays those flaws. But instead we purposely stand in our own blindspot and insist "no. THIS time I'll be different" completely forgetting the countless times that was said previously.
Long to short, our certainty is likely overstated. It wouldn't be the first time either.