Several times I sub'ed MC who made few mistakes, while the other MC was never in comments, but suddenly the MC whom I sub'ed was rated 7 or 8 at the end of the game and the other MC was rated 5...
Same. Comments etc are a guide but we just don't get enough of a picture of what goes on to be definitive.
Quite often I put my changes in place only to see the guy due to come off scoring before the ref finally allows the change.