That the cost of genetic engineering is not worth the benefit.
Which is reflected in various stories across Star Trek. They use it at the most basic level, but recognize there is a line because in the past it caused far more harm than good. It's not hiding in a corner when you recognize that this causes a very negative reaction. It's not hiding to recognize a limit.
Completely agreed. As Janeway said in "THE OMEGA DIRECTIVE", there are some lines that should never be crossed.