Answering only for how I would use this:
This would be run on hosts I log into less often, or that are used rarely and should be upgraded on boot/resume after a period of time. Previously, I had one or two of those with the auto upgrade service enabled, on channels. I disabled that when everything moved to a system flake.
It’s why I haven’t used any of the various deployment tools (though bento seems interesting), because they’re mostly push style, and this use-case needs something more pull based.
I, too, had missed that there was a flake option for the autoupdate service. For me, the idea here is that I update regularly on my active desktop, and push those revisions including the locked flake inputs to the repo. So those autoupgrades update inputs via the git pull of the lock file, and only update to revisions I have already used and built, rather than updating their lock file locally. This also means the content will already be in the store of another local system.
I might (if I can be bothered) even keep a separate branch for the known-good updates. That’s still less branch maintenance than I used to do before consolidating a common base and per-host branches into a single flake branch.