What Mick says. And to me, many UE on paper are actually 10% - 20% forced. I go to ball but a little bit rushed, or ball is slightly out of the strike zone. Of course, I can just push it back. But if I want to hit it with some interest it may end up as an UE. It is simply because not all shots are 100%. If 10% are mishit due to over hitting, with too much or too little spin, they are all UE.
Zero UE may not be a good thing.