Yeah, in this case the service doesn’t actually fail, and furthermore it seems to actually do a switch even when there are no changes… so a bit more work might be needed generally.
Edit: it’s worse than that… even with --refresh, nixos-rebuild runs, fails to fetch a new revision from git, and then builds and switches to the old revision. Which can result in a rollback if the current system was built from a newer revision in /etc/nixos than what it has cached from the last time the autoupgrade ran.