Getting my backup infrastructure to behave they way I'd want with filesystem snapshot (e.g. zfs or btrfs snapshot) was not trivial. (I think the hurdle was my particularity about the path prefix that was getting backed up.) write once pg_dumps could still have race conditions, but considerably fewer.

So, if you're using filesystem snapshots as source of backups for database, then I agree, you _should_ be good. the regular pgdumps is a workaround for other cases for me.