Some devs of some pretty important coins seem to refute this claim.
There is really only one "important" coin and that's Bitcoin, maybe. It has something like 95% market cap share (though it only a minuscule near-zero market cap share compared to fiat banking). Its developers have certainly not refuted the claim that PoS is a sham when it comes to any sort of real long-term decentralized security.
The rest are long-shot attempts to become important, with varying degrees of scamminess and developers who are varying degrees of full of shit in promoting their coin/agenda/sham. That certainly includes both of the coins being discussed here.
Important in terms of alt coins i should have said.
So POS is a sham? it's as clear cut as that? can you explain why? does it not help mitigate some of the vulnerabilities of POW only?
You are talking about complex systems with complex analysis. This is complicated by the fact that every PoS system is slightly different and PoS supporters can, and do, make subtle changes to their systems when presented with flaws, or order to "fix" the flaws. In formal and security analysis, any change, however subtle, means the analysis needs to be completely redone. Obviously you can see how this might make it infeasible to keep up with every new variation and show how each and every one of them are broken in specific detail.
Nevertheless it is possible to analyze these systems in broad terms and reach conclusions in terms of general principles, such as needing to consume some external resource (i.e. proof of "work", broadly) in order to reach a decentralized consensus.
This is explained by a Bitcoin developer here:
https://download.wpsoftware.net/bitcoin/pos.pdf"We showed that by depending only on resources within the system, proof of stake cannot be used to form a distributed consensus, since it depends on the very history it is trying to form to enforce loss of value"