You can't just generalize all PoS as being the same code and execution. ETH is quite different from the rest. Furthermore, none of the other blockchains have even close to the level of value (not just $ amount, but also fame) tied up in them.
As for rollback... that's what created ETC, but this time, it is different... with PoS, the 'stake' is tied up in the network. It isn't external, like it is with PoW and isn't as easy to just fork. You're effectively now forced to convince everyone to follow another canonical chain, but you don't have an external way to do so. Forking becomes a lot harder. It also becomes a lot more complicated and hasn't even been done before... how much 'testing' has that gotten?
As for rollback... that's what created ETC, but this time, it is different... with PoS, the 'stake' is tied up in the network. It isn't external, like it is with PoW and isn't as easy to just fork. You're effectively now forced to convince everyone to follow another canonical chain, but you don't have an external way to do so. Forking becomes a lot harder. It also becomes a lot more complicated and hasn't even been done before... how much 'testing' has that gotten?