Stock valves are a friction welded two piece affair, the hardened chromium plated shaft is spun in one direction really fast and the stainless head is spun in the other direction really fast and when they are run together they weld themselves together. It actually works pretty good, good enough to use on some performance valves with great results (although there are one piece units available from many places) but there are limitiations. While the OEM valves are excellent they are not the best, and high rpms with boost the heads can pop off - sort of like every so often a factory rod breaks or a sleeve cracks at a mundane power level. Even with great QC and OEM quality nothing is perfect.
I've seen stock valve heads pop off before, but it was on an engine that was fed a lot of boost at it's (thermally induced) knock limit, at super high rpms, with super stiff valve springs.
Frankly, I've only seen Supertech fail in commonplace nothing special use and I've seen Ferrea 6000 take insane abuse with no failures. There are a lot of other valvetrain manufacturers out there, some obviously bad (BLOX/Omni, for reasons already mentioned), but I've only experienced a few at the extreme limits. There could be (almost certainly are) Ferrea-level companies out there, but since I haven't experienced anything of their product, nor did I have some mojo laid on my like Zeke @ Ferrea laid on me, they don't get equal time on my soapbox.