Usually only about 10-20% of the coin actively engage in mining PoS.
That in itself is "worrisome" as presumably it means that forks could appear more often.
The other problem with PoS is that those not mining could be in fact colluding to present a new better chain (the cost of doing so being very little as the "Nothing At Stake" point made).
Again it comes down to "distribution" with PoS that basically *cannot be anonymous* in order to be trusted.
I'm not sure where this PoS "collude" concern come from. Who would collude to destroy their own wealth? I hold some peercoin, if you tell me to collude with you to attack Peercoin, I would tell you to gfys. Do you think it's easy to collude with 10,000 peercoin holders? or is it easier for discus fish and ghash.io to collude and gain 51%? I think the answer is obvious.
The fact is forks has NOT appeared more often in PoS, in fact PoW system has had much more forks, including Bitcoin.