Pos has many many vulnerabilities already discussed in this forum, sure it has advantages by blocking the 51% attack, but if the market cap is small then if some1 buys up 51% of the coin supply they can perform the attack easily, so since because of this a few guys will own 51% of the coins in the start era to make sure nobody can aquire more than 51% fast enough.
This makes all POS coins unevenly distributed, and basically a pump and dump because if lets say 10 guys own 51%, then what if the market cap reaches 1 billion $? You bet that all of them will sell it and buy some sports car with it, they dont care about the coin.
So because of this its hard to find a long term POS coin. I think NXT is a good coin, but it needs more features though.