I’ve been running OPNsense on Proxmox for years now, it just seems to plug along. I run ZFS for the datastores and do a snapshot before updates, but I’ve never had to use one.
Recently got it working with HA and inadvertently tested it by having a drive failure on my primary node. I remoted in for for something else and realized it had failed over to the second node about a week before, and I’d never heard a word from the family about internet being down.