btrfs, by their own account (see the btrfs wiki), isn’t safe with degraded mirrors (which kind of defeats the purpose of mirrors) or parity RAID in any configuration. They’ve written their parity RAID code with a write hole. Inexplicably, parity isn’t checksummed on disk. It’s so bad that RedHat has even dropped it, after being one of its big champions for years. Its big benefit over ZFS is that, if the stars align and you recite the proper incantation (and don’t forget the correct tongue angle), you can add individual disks to expand the capacity of a parity RAID volume, which ZFS doesn’t currently allow. But that’s changing–there’s a new project with OpenZFS to do just that, largely sponsored by iXSystems.