There are some things that can be improved about the domain backup command. Raising this bug for backporting improvements to maintenance branches (where they'll be more use to users).
An example, lots of users seem to hit an error case where the DC being backed up hasn't initialized its RID pool. This wastes a lot of time (for them, waiting for an answer, and for the samba team that's doing the answering). E.g.
bin/samba-tool domain backup offline -s /tmp/join/etc/smb.conf --targetdir=/tmp/backup/
Creating targetdir /tmp/backup/...
ERROR(<class 'IndexError'>): uncaught exception - list index out of range
File "bin/python/samba/netcmd/__init__.py", line 186, in _run
return self.run(*args, **kwargs)
File "bin/python/samba/netcmd/domain_backup.py", line 1019, in run
sid = get_sid_for_restore(samdb)
File "bin/python/samba/netcmd/domain_backup.py", line 81, in get_sid_for_restore
rid = int(res.get('rIDNextRID'))