I use a RAID for the data but the backups go to simple single disks. My reasoning is, I already have a RAID and redundancy. And I don’t have an unlimited budged. It’d already need 2 disks to fail to wreck the RAID and then also the backup has to fail with that solution. That’s probably a fire or ransomware or a deliberate effort. Adding one more disk of redundancy would probably not change much. But It’d cost and add complexity.

Also this way I don’t need to care about buying disks of a certain size and go through painful migration processes more than necessary. I can re-use the drives with mismatched sizes and swap them in to the backup pool.