I am not the best person to discuss the technical details here, but how do you explain PoW altcoins are easily 51% attacked to death. But then PoS altcoins all avoided this fate, and most of them (the non scammy ones), works and works well. Clearly when put in a equal competition (altcoins), the PoS system came out on top in an equal competitive environment (without early start advantage etc...).
I think we'll see non-clone coins being broken after two things happen:
1. They become valuable enough for attackers to bother, and there is some way for them to cash out.
2. The attackers have some time to do what they need to do to mount an attack-- write code, deploy botnets, hack into some big exchange(s), get their hands on some early-adopter's wallet backups, or whatever.
Once the tools and techniques are developed, then I think we'll see what we see in PoW 51% attacks: attacks against even mostly-worthless clonecoins, because if they've already got the tools then they might just attack for the lulz.
I'm surprised you count peercoin a PoS success-- they're still running with centralized checkpoints, aren't they?
In peercoin 0.4 (current available version), yes still checkpoints. Though Sunny King has stated in 0.5 (future version coming out soon, hopefully), checkpoint will become optional and user can opt out checkpoints. Btw, at least 90% PoW altcoin use checkpoints too, otherwise they get instantly 51% attacked to death.