It harms decentralization, transparency, verifiability and security of the network, that's why Bitcoin still stand with the current network instead of migrating or adopting other consensus. Most Bitcoin holders value decentralization and security over scalability.
Vulnerability
Furthermore, as opposed to a blockchain like Bitcoin where blocks are being continuously mined in ‘unison’, albeit opposition, by miners, with a DAG hashing only happens when processing new transactions. Malicious actors only need to gain over 33% of total hash power to be able to attack the network, even before it is sharded, and the lack of constant mining (IOTA, for example, currently processes between 1.2-2.4 tps, the vast majority of which are empty transactions) plus the minimal level of transactions makes it vulnerable to an attack.
Secondly, there is no otherwise verifiable and guaranteed list of transactions in timestamp order. Unlike blockchain, which has a block number/verifiable time of block creation, DAGs do not have guaranteed and secure timestamps, as latency/transaction execution time will vary across nodes.
This causes issues, not just for double spend but also for any application built to run on the DAG that requires an exact timestamp.As it is, decentralized security is traded for performance.
