Top post in this thread is about btrfs is on Tumbleweed, which is not an old kernel. My experiences have been similar. Btrfs is a great backup stress tool. I have no idea how Facebook is running it in production…
It's very common for MegaTechCos to hire maintainers like this, but us in the hoi polloi need to be cautious not to jump headfirst into a situation where we can't call upon the experts/inventors to get us out of any jam. We have to rely on our own abilities, the mailing list archives, and hopefully some relevant random blog posts that aren't too outdated.
This should be taken into consideration much more often than it is. If anything, maintainer employment by a MegaTechCo should nix out their implied endorsement of the technology rather than taken as proof that it's viable.
As far as why, https://code.fb.com/open-source/linux/ has a nice quote:
"Btrfs has played a role in increasing efficiency and resource utilization in Facebook’s data centers in a number of different applications. Recently, Btrfs helped eliminate priority inversions caused by the journaling behavior of the previous filesystem, when used for I/O control with cgroup2 (described below). Btrfs is the only filesystem implementation that currently works with resource isolation, and it’s now deployed on millions of servers, driving significant efficiency gains."
(I don't know though)