It solves the nothing at stake problem by comprising through it necessitating trust in some third party entity. The third party in this case is the community and it's elected witnesses. Witnesses certainly have something to lose if they misbehave, but if the community doesn't really care a lot about the witnesses then witnesses may be able to get away with some things. We trust that enough people care.
The current blockchain is vulnerable to voting collusion among whales (less than 51%) and general apathy of the majority of the users (some significant %). There are certainly small fixes that we can use to make it more secure, but that flaw with always be there. But hey, it might be worth it for such a fast and expansive blockchain.
POW requires trust in the community of block signers also, which in most cases means a few large mining pools and farms. Delegation and apathy are not unique to proof of stake, and it's best to design the protocol to account for realistic expectations about the role such factors will play.
Obviously in a POS network the distribution of stake is critical, and the distribution of Steem is quite centralized. That fact encourages even greater voter apathy in this network, but does not necessarily indicate a flaw in the consensus protocol itself.
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