Remember that this is backup. So if the backup system fails you have options like...
Taking a new backup from the live systems.
Offlining the limping array and taking a full backup of it before attempting a restore
Doing a backup/restore rather than an array recovery
All of these things make RAID 6's risks minimal. This isn't the only copy of anything, it's a backup. And it is not subject to availability risks (at least not in the way that live data is) so things that cause availability issues are not significant.