Did Drew Brees underachieve? How about Peyton Manning with Indianapolis?
Both only won one Super Bowl.
Peyton Manning was a shell of himself with Denver and was carried by a great defense.
To put this in perspective, if you go back to the beginning of the salary cap era in 1992, if your name isn't Tom Brady, the most any QB has appeared in a Super Bowl with the same team is 3. Otherwise, Peyton Manning has 4 appearances for a career.