if I have a master and backup(s) controller in my domain, where should I install nextcloud “app”? On master? Or can I deploy it on one of the backup DCs?
In general I try to keep both my DC Master and DC Backup as free from other apps as possible. That ensures updates to those critical systems are easier to do & less prone to failure. If at all possible I install apps on member servers or slave servers (some apps cannot be installed on certain server types).
The background is that a domain’s DC Master must always be the first one to be updated. If the DC Master cannot be updated, e.g. because an app is installed on it but that app isn’t available for the new UCS version yet, you cannot update any of your other machines either. Similar reasoning applies to your DC Backups: if you ever have to promote a DC Backup to be your new DC Master, and you have apps installed on that Backup-to-become-Master, you might run into the same problem.
Having the apps on a member or slave server doesn’t pose the same risk as you can safely operate servers with different UCS versions within the same domain as long as your DC Master always runs the newest version. If you have a member server with an app that blocks the update, you can update your DC Master, your DC Backup, your other member servers and keep that one member server running on an older version of UCS for the time being.
 Within reason: UCS 4.4 + 4.3 should be OK, but that doesn’t extend to arbitrarily old versions such as 3.x. If in doubt, consult the release notes when planning your updates — which you should do anyway.
ok, makes sense, thank you for the clarification!