If the majority of work is controlled by honest nodes then the chain is safe. Not done by honest nodes, controlled by honest nodes.
With PoW an attacker might produce new miners and gain control of the network.
With PoS unless enough people sell to him their money, a "new" attacker just can't do anything.
(not implying there might not be other attack vectors)
1) Exchanges can and will be hacked if its worth it, see Vericoin and NXT just for an example of the last weeks.
2) Exchanges don´t even need to be hacked, a drunken worker there might just do it for fun and profit.
3) If exchanges get a nice letter from certain 3 letter agencies or just a visit and servers confiscated they have your coins too.
Both of those are nearly zero cost attacks and some people might just do it for the sake of it, because they can do it.
In case of PoW coins there would be value lost, in case of a PoS coin the whole network is endangered.