I also don't really understand how being correct under physical branching with ZFS, or physical backups of a filesystem, are different from crash safety in gene

I also don't really understand how being correct under physical branching with ZFS, or physical backups of a filesystem, are different from crash safety in general. As long as you replay the WAL at the point where you branch (or take a physical backup of the filesystem) you should not lose data? At the same time Postgres people don't seem comfortable with the idea in practice so I'm not sure if this is actually ok to do.