I've been recently testing Urbackup with btrfs, since my ext4 partition of 8TB and a gazillion files seemed to cause some issues for Urbackup. Results are dreadful...
It performed well, for a week or two. And then, after some snapshots (I have 35 mailservers backing up), it's terrible. I've tried different mount options, but nothing seems to help.
Now, there is ZFS, which works far better and is more stable than btrfs. It only lacks reflinks. But why is a reflink so important? I could care less if reflinks are enabled or not. Storage is not that expensive, performance is.
So please, implement ZFS snapshots, disable reflink if you detect ZFS. Both btrfs and ext4 seems unwilling for larger installs..