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Stopping Power is only 'OP' or 'unbalanced' in the extremely artificial situation where players of identical skill see each other at the exact same time in a one-on-one engagement from positions of identical cover.
So in reality it's not unbalanced or OP at all.
Also, CB wasn't underused in MW2 because it was inferior - its just that seeing "increased damage" as a perk effect for SP proved too tempting for most people.