What is truly wrong with individual drives representing individual volumes with manual redundancy only on data needing it?

For instance, let’s say I have a 20TB, a 12TB, and an 8TB drive for a 4-bay NAS, then most RAID setups don’t make sense. Additionally, let’s say I really only have <8TB of data needing redundancy too… So, in this case, is there a real reason to not setup as three individual drives, with manual redundancy of necessary data; like a nightly rsync, etc.?

I get why RAID 5, say, is the most recommended, and also understand that RAID isn’t a true, completely safe backup, nor is the setup described above.

Just curious for perspectives from longer-time datahoarders. I’m a newbie to multi-drive NAS hardware.

(Also, is the scenario I described above even possible on, say, QNap or Synology; or is it only RAID or JBOD?)