Proof of stake does not offer any resistance to Cybil attack or (part time) collusion of stake holders.

In the case of such attacks the victims will find themselves in a situation that can be described as "shadow ban" - their transactions are not accepted or accepted too late, etc.

Proof of stake doesn't prevent 51% attacks. It eliminates the incentive. If an attacker controls 51% of coins, why would they attack the network? That would be attacking their own wealth.

Not if they can attack the network without anybody knowing. If an attacker secretly controls 51% of the coins (by, say, having three separate entities that each control 15-20%) and can manipulate the market in their favor by rejecting transactions that would hurt them, that's a big incentive.