People who have been running btrfs in production (reckless, imho) on kernels that are 2.5+ years old or worse. And then they're surprised and disappointed when it loses data.
For everyone who's posted here with a btrfs data loss story, it would be helpful to quantify your anecdote with what kernel you were running.
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)
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to rely on our own abilities, the mailing list archives, and hopefully some relevant random blog posts that aren't too outdated.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.