I don't have an AR pistol, however I do have an AR15 (SBR) with an 8 1/2" barrel on it, so I can still probably answer some of your questions. I have owned it for 4 years, and no breakages.
Contrary to a lot of popular belief, they do not have to be excessively more prone to wear or breakage than any other AR15, IF they're built right.
And to be honest, on barrels shorter than 10.5 inches, they have less dwell time than a carbine length gas system, which is good because the gas is hotter.
But like I have said, I have not had to replace any parts so far. This includes the gas tube and extractor, which some folks claim to have to replace at earlier intervals.
The main thing you may need to consider is the distance you're going to be shooting because shorter barrels do not have as much bullet velocity, which really shows at extended distances. Up close, you probably won't be able to tell the difference, as this video shows if you watch and listen to it.
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