It's stable on stable hardware for some time. The multiple device stuff has missing features mainly related to error handling when a device starts to go crazy,

It's stable on stable hardware for some time. The multiple device stuff has missing features mainly related to error handling when a device starts to go crazy, and that flat out requires a sysadmin who understands all of that. i.e. Btrfs won't consider a block device unreliable and just ignore it, it'll keep retrying to read or write, while filling up the system log with all of the errors. When there's redundancy, it does fixup these problems automatically, but it can drown in its own noise if a device is producing an overwhelming amount of spurious data. And there's no notification system for this: but note there's no standard notification for this on Linux at all either. LVM and md/mdadm RAIDs do not share the same error handling or notification. The main issue for whether it's the default filesystem is whether the distro has the resources to support it for their users. Mostly this is in terms of documentation, and understanding what sort of backports to support. Really the only distro doing that work is Suse and maybe Oracle. I don't expect the more conservative distros to support it for some time until they feel they can depend on upstream's backporting alone.