In production I don't use any vfs cache on my union yet...
I think the union should have built-in failover yeah...otherwise what's the point?
It should work like this: file is opened, it will follow the union policy, in my case it's random...so it will randomly pick a remote and try to open a file, if that fails for whatever reason, it should call another random remote, and only give up if all remotes fail...
It would be sensible to mark failed files in the remotes as "dead" for 24 hours. I'm still having lots of 403 errors, mostly because rclone is not load-balancing between my remotes, so I would be able to test any branch and tell if it's working as intended or not quickly...