What happens in case of a hard fork? How will they ensure that no one will validate blocks in the wrong fork? With PoW the miners much choose one fork or at lest split the hash power. With PoS the miners can validate blocks in both hard forks without too much effort.

Moreover, if the 51% dislike the one fork, or the version before the fork, it's too easy to validate correctly in the part of the fork they like and launch a 51% attack to the other fork. This would have killed ETC and BCC instantly. (I'm not sure if this is a bug or a feature.)