Flipping a safety on inadvertently during the draw can be a serious problem.
Again, no one is suggesting mixing up opposing platforms on a daily basis, so it's pointless to pursue that idea.
Total round count
alone does not equal mastery of a given platform. If you fired 200,000 rounds thru a given gun model, how much did you improve from 190,000 to 200,000? Or even those last 50-100,000 rounds?
Some folks have their "skills" peak at 100 rounds because their technique is poor and they don't think it's necessary to seek out quality instruction.
As often happens, people are getting caught up in words like "mastery" and "proficiency." I like to define these things, but then folks get caught up in the definitions offered.
I mean you're able to instantly operate any given platform correctly, safely and competently.
For example, YouTube has videos of a guy discussing snubbies, but the opening sequence shows him taking 4 seconds per shot because he hasn't mastered basic DA shooting. That guy doesn't know enough about them to discuss them intelligently.
Or someone shooting a Sig 226, but doesn't decock the gun before holstering it.
Friend of mine bragged he didn't need to practice with his BHP, as he was already good enough to "cut a man in 1/2" with it. When I got him to the range, he didn't know which was the mag release, slide stop or safety.
BIL keeps a Beretta 92 loaded for home defense, but hasn't solved the DA issue, even tho it's a pretty nice pull.