Due to its energy efficiency and scalability features, Proof of Stake has been adopted far and wide, even though there are various issues regarding it that daunt decentralization, security, and the network governance process.
One of the major issues is that there could be a possibility of centralization; because selection among validators is done in correspondence to some function of the number of tokens they have staked, richer participants would get selected more often and get paid for the same. Thus, power-and hence, wealth-becomes concentrated in the hands of a few, which in turn reduces decentralization.
Although it is less feasible for an attack against a PoS than the Proof of Work, the possibility does not get entirely removed. He would need to keep 51% of the staked tokens in his ownership this time and would let him dominate the network. Although prohibitively expensive, he may manipulate any transaction flowing across the blockchain or bring it to complete paralysis.
The initial distribution of tokens might make a difference in decentralization. If some large early adopters hold most of the tokens, it's easy for these holders to dominate the network and discourage the participation of smaller or newer validators.
Slashing is one feature of significant security that turns counterproductive and makes validators wary of experiencing it due to the fear of losing funds for accidental mistakes or minor infractions.
These challenges give a certain view on the difficulty of realizing full security with a completely decentralized PoS system.
Thanks
~ Nesaty