I've discussed my various doubts about the delegated Proof of Stake a few moths back. Thinking about it some more recently, I have one more idea to add to that discussion that applies both to Proof of Stake, and distributed PoS - the lack of a fail-safe.
People criticise Proof of Work for creating centralization. Bigger miners earn more money that they invest in faster mining equipment to earn even more money and so on until there is only one entity that essentially owns the network. Everyone fears centralization in Bitcoin as it can bring about the dreaded 51% attack. However, a lot of people, including myself aren't really worried about this. Andreas Antonopoulos explained it well during the Texas Bitcoin Conference - a 51% attack isn't much of a threat any more since we can always fork the software and make the current ASICs obsolete. Just like that, any mining operation that relies on those specialized chips have it in their best interest not to attempt to be a malicious entity - they have too much to lose. While a malicious "government entity" (or anyone that wants to destroy the network at a cost) would not case about the losses, it wouldn't be able to accomplish anything regardless.
Looking at Proof of Stake, the network essentially lacks this fail-safe. If an entity controls the majority of coins in the system, they can perform a 51% attack as well. To take the network back, either one would need to abandon the PoS mining algorithm, or erase the malicious entity out of existance. However, due to the pseudonymous nature of cryptocurrencies, the attacker in question can easily shuffle their coins to new addresses and spread them around so much they become essentially indistinguishable from anyone else in the network. Tweezing them out would be hard and a lot of other people might get removed by a false-positive. As such, the network that relies on Proof of Stake cannot purge itself from a malicious attacker, like a Proof of Work network could.
Of course, a non-malicious entity wouldn't want to perform a 51% attack on the network. The attack would evaporate any value their stake would have. However, a malicious "government entity" that wished to take down a PoS network would have a much easier time doing it, since forking them out of the ledger would be much harder.