For backups in particular, are ZFS snapshots alone not suitable to serve as a backup? Is there something else that the pg backup process does that is not covered by a "dumb" snapshot?
That being said, wal-g has worked well enough for us that we haven't put a ton of time into investigating alternatives yet, so I can't say for sure whether snapshots would be a better option.
Also, auto-expiry of no-longer-needed WAL segments (that we use due to our reliance on async hot standbys) along with previous backups is pretty great.
And we haven't even started taking advantage of pgBackRest's ability to do incremental restore — i.e. to converge a dataset already on disk, that may have fallen out of sync, with as few updates as possible. We're thinking we could use this to allow data science use-cases that would involve writing to a replica, by promoting the replica, allowing the writes, and then converging it back to an up-to-date replica after the fact.