To my knowledge, no proof-of-stake coin has been compromised in a 51% attack
Peercoin has been successfully attacked. The reason these PoS coins are still up and running today is because they either use checkpoints (which makes them 100% centralized and pointless, which is the case of Peercoin), or they are continually making changes which makes attacking them annoying or hard, but is just another form of centralization (they depend on the continuous updates that everyone must accept blindly, as opposed to a stable protocol that rarely changes. Someone compared this to the whack-a-mole game).
Nxt doesn't use checkpoints. Although it adds features fairly often, its core model doesn't change often and new features don't make it harder to attack. Even if they did, I don't see how that would affect mounting a 51% attack. Getting to control 51% of Nxt would be implausibly difficult because of the difficulty in acquiring that many coins.