POS has its own very serious problem:
Someone with a stake can spend it to mine any number of forks simultaneously.
so this is just merged mining, eg with IXC and others in BTC. No problem here.
In contrast POW can only be created on alternatives if computing capacity is split between them. This forces convergence while POS does not.
no merged mining does this without extra hash. The hash for BTC will always work for a lower difficulty.