The fundamental problem is that with PoS it's possible to dispose of your holdings yet still retain voting power (in the past, from the blockchain's perspective), whereas the same can't be done with PoW's mining equipment.
With PoS, withdrawing your stake (read: disposing of your holdings) withdraws your ability to act as a validator. And pretty much every mainstream PoS implementation imposes a delay on stake withdrawal to give the rest of the network time to make sure that whoever is withdrawing their stake hasn't already validated fraudulent transactions.
That's still fundamentally a weaker system, no? Specifically, still possible to double spend, the only difference is that we assume that the network wouldn't accept block re-orgs that are that long.