My range facility put 1 million pistol rounds downrange every quarter for years, I've seen everything break. Everything WILL eventually break. Glocks have a stated factory service life of 20 to 30k in 9mm, and 20k (or less) in some models. Their .40 cal pistols WILL fail more frequently due to the nature of having higher pressures in a handgun designed for 9mm.
In my experience (100 Glock+ handguns alone returned to factory for repair or catastrophic failure replacement), the main reason GLocks fail after 20k rounds (and before that), is people not changing out the recoil spring assembly. When that wears out, the handgun will quickly start beating/eating itself, resulting in slide/rail separations, fractures, and all kinds of other failures. So you're right, maintanence is a major failing point, and not really the handguns fault. Still, I've seen brand new handguns of every caliber and description break/fail in their first 1000rnds. It's random, and not very likely, but it happens.
The biggest problem with the Glocks in .40 S&W is that their chamber does not fully support the case, and the barrels design is not friendly to lead and copper wash bullets, You have to use copper jacketed rounds in a Glock. Scratch +P hot loads like Cor-bon and Buffalo bore to. Handloads void your warranty.
Typically Unsupported Chamber with a Polygonal rifled barrel firing Lead Bullet Followed by +P Loads causes a glock to go KABOOM!
http://f-r-i.com/glock/FAQ/FAQ-kb.htmThey also lack a hammer which brings up a new problem that left me with a scar under my right eye. I had a factory cartridge with a bad primer fail to fire. I waited the obligatory 10 count, ejected the round and BOOOM! the cartridge went off hitting me in the face with brass fragments .... If I wasn't wearing eye protection I would be blind in my right eye.
I returned my issue weapon and purchased a Smith and Wesson 411 .40 and a Sig p229 .40, I fell in love the p229 and made it my primary duty/carry weapon.
I have loved Sigs ever since. Now I have a p220st .45 a p220 .40 and a p239 .40. I carry them all (Usually not at the same time).
Unfortunately Sig has been moving away from their p2XX designs for the last decade and I hear they are closing their European factories due to financial and legislative problems.