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?