BTRFS should be stable in the case of power loss. That is to say, it ought to recover to a valid state. I believe the only unstable modes are RAID 5/6.
I'd recommend BTRFS in RAID1 mode over mdadm RAID1 + ext4. You get checksumming and scrubs to detect drive failures and data corruptions. You also have snapshotting, in case you're prone to the occasional fat-fingered rm -rf.