The issue with PoS is that it introduces trust into the element. There doesn't seem to be a way (yet?) where you can introduce PoS without lowering the security of new adopters.
PoS works on the premise that rewards will be given to holders. Unlike PoW, PoS has a single incentive structure where the management of the networks is aligned with the holders of the tokens. If you are not a large holder, the existing holders can re-write the rules to their advantage. You have to trust that large holders won't do this, even though the incentives align with that outcome.