OK the above isn't exactly correct in the last sentence. If there is URE due to medium error then we get a bad stripe and repeating rebuild will most often achi

OK the above isn't exactly correct in the last sentence. If there is URE due to medium error then we get a bad stripe and repeating rebuild will most often achieve nothing to remove it (although see SpinRite which can sometimes recover data from bad sectors). However, even if the bad sector can't be read, the "RAID failure" is still just loss of single or few sectors, which usually isn't a big deal. The array filesystem data is then best copied to a new array on new disks and then the old array discarded.