Incidentally, the application I'm reworking to use btrfs is cocalc.com. One of our main use cases is distributed assignments to students in classes, as part of the course management functionality. Imagine a class with 1500 students all getting an exact copy of a 50 MB folder, which they'll edit a little bit, and then it will be collected. The copy-on-write functionality of btrfs is fantastic for this use case (both in speed and disk usage).

Also, the out-of-band deduplication for btrfs using https://github.com/Zygo/bees is very impressive and flexible, in a way that ZFS just doesn't match.