From my understanding, the issue with PoS is that if a chain forks you can mine both sides without penalty. A consequence of this is that if someone wants to retroactively create a fork that starting from a block a few years ago, then people could do a few years worth of mining of this fork in an instant. And it wouldn't be possible to tell these two chains apart cryptographically. With PoW it would be computationally expensive to pull such a stunt.