It's "perpetually half-finished". Bad track record with reliability and loss of data, lots of stories on the internet. It perhaps got better, but RAID5/RAID6 for metadata (standard use of disks) still not oficially recommended and thus standard RAID5/RAID6 is not reliable. Also, look at some FS benchmarks - BTRFS is still much slower than EXT/XFS, and slower than ZFS.

About the only class of scenarious to use BTRFS instead of ZFS is if you want the features, do not need RAID5/6, require support by Linux distributor/kernel developers or for some other (hard to think of) reason you can't use ZFS.