I thought the usual recommendation was to use mdadm to build the disk pool and then use btrfs on top of that - but that might be out of date. I haven't used it in a while
The md metadata is not adequately protected. Btrfs checksums can tell you when a file has gone bad but not self-heal. And I'm sure there are going to be caching/perf benefits left on the table not having btrfs manage all the block storage itself.