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
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http://www.thecodelesscode.com/case/6