It's "perpetually half-finished". Convoluted admin cli tooling, 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 scenarios 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.

I do use BTRFS on a single disk where I don't touch it with cli tools, and there it's fine.

Have you tried reading the code? It's twice as large (107KLOC) as ext4 (45KLOC), where ext4 is viewed as a natural evolution of ext2 (7KLOC).

45KLOC of C code already sounds really scary to me, although yeah, journaling is really nice to have.