The main reason I asked is because, generally speaking, BTRFS and ZFS offer a lot of the same advantages. Both allow multi-disk pools, both have data integrity features, both have snapshots, both offer a delta based method of sending snapshots from one fs to another.
ZFS pools can apparently offer faster speeds at the expense of more resources. ZFS pools are less flexible in their layout and in terms of expanding them, BTRFS is more flexible (though not as much as the unRAID array) but lacks a stable raid5 or raid6 option. In the wider linux community ZFS is considered more stable than BTRFS (opinions vary on the stability of BTRFS), but in unRAID BTRFS has been integrated for longer.