@TPTB_need_war another way to think about why PoS isn't as secure as PoW in general:
PoS does not reinforce historical consensus. Every subsequent block in a PoW chain makes the history below it more secure because the cost of reversing it is superlinear in the number of blocks built on top. In PoS, this is not the case, the cost of producing a block is a constant, therefore the cost of reversing history is a constant.
so with a 51% + selfish mining attack you would be able to unwind all hist tx in PoS? (with minor costs)
You can arbitrarily re-write history in PoS with <50%; I can produce a valid candidate chain longer than the canonical chain for a constant cost, whcih I then present to nodes which are syncing with the network who are unable to distinguish this objectively from the canonical chain.
edit: Since the cost of providing such information is very small, I can dominate the network with peers containing instances of my fake chain such that any syncing node querying peers at random would find a majority of my fake nodes.