I've had scans that looked terrible, but were readable (see first attachment). that was burned on cheap media with a very old 4x CD writer.
The second was burned on better media with a Plextor PX712a, and that result is fairly typical.
At a guess, I'd say C1 errors don't matter too much, but any C2's will cause the drive to slow down and re-read the sector a few times. Any CU's mean that sector is largely unreadable. Also, there seems to be a lot of C2 errors in the vicinity of a CU.