From my understanding (and I am no mathematician or programmer) the data structure of transactions that utilize DAG require one or more previous transactions to be confirmed and is measured in accumulated amount of proof-of-work referencing the transaction.
Byteball, which seems like one of the more popular and sound DAG projects requires 12 ‘annointed’ witnesses that validate all of the transactions. Aside from this being centralized, it also lends itself to latency issues.
IOTA is similar in that it does not work without centralized servers
DAG is still a fresh topic for me, but this sounds like a good topic for my next article: DAG compared to Hashgraph