It may open you up to Ransomware attacks that would affect multiple backups. For example, one of those machines get infected, the NAS share credentials are compromised, now the attacker has access to the infect machine's backups plus all the other machines. ZFS snapshots may make this a non-issue, but it is something to consider. It may also be non-issue if the time machine backups are done by multiple user accounts, so user A can't touch user B's backup, I haven't tested that out.