I'll be honest guys, and I'm not knocking 21st Cent. Toys (like beating a dead horse), but I've had some extensive dealings with plastics professionally and personally between work and collecting... 21st's NOT made of the best materials all the time. It's kind of easy to tell at times. Even new...
Not knocking them, I'm just saying that I won't be surprised if they really need to be carefully handled as they get older.
There's definitely some issues with the materials used though, and like I said I feel pretty sure 21st wasn't always using the highest quality plastics available. Big companies don't always even, but smaller companies seem to do this even more, as a cost-cutter in the manufacturing process.
Not knocking them, but I wouldn't be shocked is all... And sometimes, they may be fine. A different run, a different factory... You just never know. I've had some figures that, as others noted, snapped immediately. This was at times because a figure's paint sealed the joint or whatnot, but not always. Sometimes I can just tell it was an inferior plastic was used on a pin or whatnot. It happens.
Poly-vinyl has become more of a common plastic for making action figures for a few different production-centric reasons mostly. It also breaks down faster, generally. Bad poly-vinyl can turn very gummy over time, it's been known to discolor easier (especially white), and so forth. Not much one can do about the future of this stuff other than be careful with it. That's my best advice... That and keep your stuff way out of harm's way from chemicals, sunlight, smoke, excessive heat, excessive cold... well, sadly, basically anything.
