It means recovery from corruption can be impossible with encryption in the way.
Corruption can happen with hardware errors, like bad RAM, cables, or power issues. The problem is, you don't know it's going to happen until it does, and RAID (of any sort, not just Unraid) can't always compensate, meaning unless you have complete backups, you will lose data.
Unraid or any RAID can't help with file deletion or overwriting good data with bad, so backups are always needed, but with encryption, the recovery options are even more limited, so backups are even more necessary.
If the data is important enough to encrypt, it's important enough to keep multiple copies in multiple locations.