Proof of Stake has proven quite robust. None of the larger PoS coins have been compromised (despite much theoretical discussion), and newer ones such as Cardano seem to address early objections. Peercoin has been around for a while without any issues.
The fact is, I see all of those massive distributed consensus algorithms as much more robust than they appeared initially. 51%/double spend attacks in practice are extremely hard to occur because of the exposure/publicity of the ledger and how hard it would be to capitalize on crashing the currency (PoS/PoW does not change it).
They're not mathematically provable and the jury is still out. Attackers who know some exploit might be waiting for the much larger Ethereum to switch to PoS so they can reap maximum profit.
The fact is, I see all of those massive distributed consensus algorithms as much more robust than they appeared initially. 51%/double spend attacks in practice are extremely hard to occur because of the exposure/publicity of the ledger and how hard it would be to capitalize on crashing the currency (PoS/PoW does not change it).