I would say poorly designed.

The typical btrfs fix is (slightly exaggerated) "When doing an A while a B is pending, re-enumerate the Cs for the purpose of D unless the E is locked, in which case, reschedule the F". Where A...F are all not trivial like snapshot, out of disk space, device rebalancing, and so on.

If that level of thinking of all the things at once is required to write correct code, well, it's evidently not happening.