And if you can't control growth, those unslugged pops will keep multiplying.
Also affects the traits gained from the alien box, the enigmatic cache or self-modification and such.
You can only apply a template with these traits to pops that already have that, so every event while you have a colony ship on the way splits genemodding your species in categories.
So depending on how your game goes you can have to manage half a dozen subspecies now.
And it also removes the ability to modify out of restricted habitabilities from origins, while migration pacts and robots are still easier to get.
Frankly I dislike the change. It creates more micro, more inconvenience and has no positive side.
Most of these traits are already miniscule in effect so stacking them was not a balance issues, and combining the ascensions is enough effort that a few percentage points more output are a nice non-gamebreaking reward for it. The leader traits are explicitely programmed to not stack already.