Well, wearing out the slide catch isn't really a propane related issue as it's only loaded by the recoil spring instead of the propane blowback.
The safety wearing is also pretty lame. The slide-frame shouldn't have interferences either with duster or propane. Is it possible you got a bad GBB?
Most KSC GBBs seem pretty good. The USP sounds like the first one with chronic issues and actual design flaws. It might actually be a bad build. Their Glocks certainly do better even with propane.
Want nearly free GBB gas?