1. Since the chosen single node for each block is deterministic, then in theory it could be very vulnerable to botnet DDoS attack. More generally it lacks fault tolerance, which is critically needed in real world systems. A redesign to have simultaneous disjoint blocks from multiple delegates can't be allowed because there could be double-spends in the presumed disjoint blocks.

2. For DPOS, this is not decentralized control, because the minority has to accept the will of the majority on the election of delegate DPOS nodes, i.e. the permissionless attribute can be lost such as ChainAnchor being planned for ButtCON. No one can just standup a full node at-will. This also means there isn't really competition in terms of a free market rate for transaction fees.

Btw, Bitcoin-NG accomplishes basically the same deterministic node per block and with decentralized control over the selection of the delegate in real-time employing PoW, yet with chain reorganization issues.

3. The maximum speed (minimum delay) of confirmations is lower bounded by the slowest latency of block propagation to every DPOS node, because otherwise some nodes can't keep up. This can be reasonably fast say several seconds if you've got a very organized set of delegate nodes (but then you really don't have decentralized control), but this is not fast enough for some types of instant microtransactions.

4. Zero-confirmation double-spend transactions (aka Finney Attack) are even more plausible, because a colluding delegate node knows deterministically when it will win the block. Note block periods can be reasonably fast in DPOS, so 0-conf is probably not needed although not fast enough for some types of instant microtransactions, although such probably wouldn't be Finney attacked due to their small values.

5. Proof-of-stake is not a secure consensus algorithm, because for example the nothing-at-stake problem. We compiled a laundry list of flaws in proof-of-stake. Note I recently made a suggestion to jl777 and we mutually designed how to record check points for DPoS coins in a PoW block chain.