[tromp]

The main disadvantage is that their growth rate is uncontrollable. If it's possible to add billions of tx to the DAG in a day (possibly with some custom hardware for solving simple PoWs), and a node needs to process those billions of tx to have absolute confidence that some payment it receives is not double spent, then the system breaks down.

[dustfinger]

Would a tx rate limiter not help here?

[tromp]

How do you limit the rate? I only see the requirement of a low-difficulty PoW, which is easily subverted with some custom hardware.