No matter which algorithm, in order to determine which blockchain is the correct blockchain, Proof of Stake blockchain users need to trust that they have been told the correct initial account balances. How can you prove in a decentralized manner whether you are mining the correct blockchain? You can't without outside help. This makes Proof of Stake practically useless.

And along the same lines, POW-POS hybrids are only as strong/decentralized as their Proof of Work component.

Agreed?