Well, having been to many LE armorer classes for a number of different firearms, my perspective is probably a bit biased from having been taught how to diagnose and correct or repair a range of problems.
I've corrected or repaired more problems than I can remember over the years. There were a small handful of problems that required returning the guns to the company for repair.
Sure, sometimes it might take a time or two (or more) in order to completely diagnose and effect a repair or correction for some particular gun (especially if more than one underlying problem existed, or contributing conditions were involved).
The only personally-owned handgun I tired of trying to get to function properly was an early CA Bulldog .44 Spl. I think it was after the third attempt by the company to fix the same problem (cylinder seized during dry or live-fire), I traded the gun in at the shop who had been handling the repair shipping for me, and who apparently had a bored smith looking for a challenging project. I was younger then, and was probably a bit less patient when it came to letting someone try to solve and correct problems.
I've had no qualms carrying and using repaired guns since I'm been working as an armorer.
When it comes to confirming repairs, I've commonly heard from some of the big gun companies that they recommend test-firing to confirm normal function using anywhere from 1-3 magazine loads (for pistols).
As an armorer, if I have the time and extra ammo available, I like to give the issued user or owner the opportunity to test-fire up to 50 rounds to satisfy themselves that their guns have been repaired and restored to normal functioning. (That's after I've been satisfied it works, or another instructor/armorer has test-fired it for me.)
I often tell the user/owner that I won't knowingly return a gun to service until I'd be willing to carry and use it, myself.
It's probably also important to mention that a surprising number of "gun problems" actually turn out to be "shooter" and/or "ammo" problems, too.
Those can sometimes be harder to diagnose and correct, especially if the shooter/owner aren't willing to accept
they're somehow involved, or are fully responsible for the "problem".
Naturally, it can be annoying or frustrating when a new gun design/model turns out to have some quirks, defects or teething pains. More so if a gun eventually has to be replaced with a revised model, instead of just being able to be repaired with upgraded/updated parts. That can make some folks understandably a bit "gun shy".