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the usual recommendation was to use mdadm to build the disk pool and then use btrfs on top of that - but that might be out of date.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

This is very much a big compromise where you decide for yourself that storage capacity and maybe throughput are more important than anything else.

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.