so they already decided to take a simple concept (recursive quotas applied to subvolumes) and break it: https://github.com/kdave/btrfs-progs/commit/a26bfde051dd5cfdbade7dcaaeb0df770aead6d0
- The qgroup implementation is considered reasonably stable for daily use and has
- been enabled in various distributions.
- The qgroup mode is considered not recommended for daily usage, unless there is
- no planned new snapshot/subvolume creation and deletion.
that is reflected on their docsite, but it's written into the quota command, which has absolutely nothing to do with qgroups: https://btrfs.readthedocs.io/en/latest/btrfs-quota.html#stability-and-performance-implications
