In the case of five disks of the same capacity, RAID6 or RAIDZ2 only gets you 20% more capacity than btrfs RAID1. That's not exactly a huge disparity, usually not enough to be a show-stopper on its own. There are plenty of scenarios where the features ZFS has which btrfs lacks are more important than the features that btrfs has which ZFS lacks. My point is simply that btrfs RAID1 has its uses and shouldn't be dismissed out of hand.