Centralization
PoW High risk of centralization mainly due to the reason mining being a expensive process and therefore reserved predominantly for large scale operations. This is contrary to the main principle advocated by blockchain technology which is the concept of decentralization
PoS A fairer concept in which the amount of network control granted is directly proportional to the the amount invested in the token/coin.
A couple notes on this point.
The degree of centralization is more complex than this. In POW, you can have mining concentration, centralization of mining chip manufacturing (see Bitmain), node centralization (lack of full nodes).
In POS, you can have node centralization and staking centralization. Regarding the latter, the closer to a majority of network validators that any one group has control of, the easier it is to mount an attack on the network. "Fairness" has nothing to do with centralization -- that is a separate concept regarding supply distribution itself. POS systems often have highly concentrated supplies, which is inherently unsafe for investors.