So, apart from the guys shouting "rich will get richer"
It's not that. It's the rich will have untouchable anonymous power. There is no way to stop a cabal of people from taking permanent control and reversing transactions discretely.
But with PoS they need 51% of the entire wealth of the currency, to do so, and at that point why would they do it? they will only undermine themselves when discovered because the value of the currency will plummet.
With PoW, they only need about 10% of the wealth to overwhelm the mining hashrate, because the current Bitcoin mining operations are worth about 10% of the Bitcoin marketcap.