Another thing that is worth knowing about:
Veeam performs data validation on the fly while copying the data from one site to another, and DD just copies the data without checking if the backups are any good. In other words, using DD replication is not recommended, to say the least
So I think that at least primary storage should be a general purpose server (for the sake of a better performance of backups/restores to/from the primary, "hot" backup chain). The secondary storage can be DD so it saves you some space on copied chains.