POS has its own very serious problem:
Someone with a stake can spend it to mine any number of forks simultaneously.
In contrast POW can only be created on alternatives if computing capacity is split between them. This forces convergence while POS does not.
If you examine the
designs listed in the wiki, you'll see they're both resilient to this.
In my system, if a stakeholder signs two conflicting blocks, evidence of this is referenced and the voting weight of his address is reset. (Moving to a new address also resets voting weight, until it accumulates weight again).
The same cannot be said about the alts that pass for PoS these days.