On the btrfs mailing list [1], there are still sporadic reports of unrecoverable FS corruption for whatever reason. See [2], [3] for some recent examples.
[3] is interesting, thanks for linking that. [2] has so many moving parts, I wouldn't expect it to be related to btrfs without more information. I mean, there's both the fs and the cache layer being resized down, with unknown method.