I've had 3 really bad kernel downtimes over the past 15 years or so, and 2 of them were due to btrfs quotas.

Quotas that didn't work, or quotas that worked perfectly fine, but blew up your system in an unexpected way?

The reason I ask is because I'm trying to tease out if you have architectural problems with the way the filesystem is designed, or if you simply think it's unreliable.

I never wanted quotas at all, but a systemd tool turned them on unbeknownst to me. That was one problem. The second and worse problem was that the performance with quotas on was so terrible that the machine bricked.