You can add/delete devices from raid5/6 volumes now. The raid5/6 code is still experimental, in particular while detected problems are fixed on-the-fly to userspace, the fixes aren't written back to drives. That limitation applies to normal usage and scrubbing. A balance detects and fixes these.

Also the determination of a drive being "faulty" (in the md/mdadm sense) and how this gets communicated to userspace isn't in place. If it's in place for ZoL (?) that'd be a considerable difference, a bigger one than checksum algorithms in my opinion.