Most.
I don't think I could put a percentage on it because the more milled solutions the more we'd find the issues with those. Of course, I believe the issues would be way less common. I think milling for the individual optic is about as good as one can get in preventing mounting failure.
Obviously with milling you're a bit married to the footprint, it modifies the pistol, and it can always be done wrong (I know people who have had slides messed up by incorrect milling jobs. That said, it was always made right one way or another).