What happens when a URE is encountered during a RAID rebuild?

My current NAS (Synology 8x10TB) is near capacity and I’m looking to buy another one (8x22TB) ASAP.

After reading an alarming amount of articles when purchasing the first one about how RAID 5 was obsolete, I opted for RAID 6, but with hard drive capacities more than double the original size, I’m even doubting the validity of RAID 6.

I was wondering what would happen when a URE was encountered during a RAID rebuild. I found another SuperUser question What happens when a RAID rebuild fails due to URE? which asks the same question, but the accepted answer states that the entire RAID is lost (and thus all contents). On the other hand, I often read that the RAID will still be rebuilt, but that certain blocks will be marked “unrecoverable”, meaning that one would only lose an amount of files.

Which of the above is correct? Do you actually lose the entire RAID, or just an amount of files? And is the answer the same for all RAID controllers? If not, what would the answer be specifically for a Synology NAS?

On a side note: yes, I do understand that RAID is not a backup, all data on my Synology is backed up to another Synology, which in turn is backed up to separate hard disks. So I do have the backups, I’m just not looking forward to restoring any of them…