How vulnerable is proof-of-stake?

I read in a white paper of a cryptocurrency, forgot which one, that mentioned that a draw back of POS is the ability for an address or group of addresses that previously held a majority of the coin to be able to collude and make new blocks. Proof of work should only be able to be exploited if the majority of hashing power at the current time. Does the consensus mechanism only allow a certain amount of blocks to be appended within a time interval?