Why does a URE cause loss of array during RAID 5 rebuild?

From what I have read, RAID 5 is problematic with large disks becuase if a single disk fails, you are likely to have an unrecoverable read error while the array is being rebuilt. From what I can gather, this URE prevents the entire array from being rebuilt. Why does an error in a single bit/block/sector cause the whole rebuild to fail?

In terms of worse case scenerios, I could image if the URE occurred in a "bad" place (e.g., a filesystem superblock) you could lose everything, but do you always lose everything and if so why?

Solution 1:

When faced with known inconsistent data, you don't run with it and hope for the best. If you reach the conditions your RAID level, by design, cannot tolerate, you stop. This is what your backups are for and precisely the conditions under which you already understand you're going to use them.

RAID is not backup. It's a way to continue running through a certain class of failures.