It helps to look at things this way:
Romo - 18 fourth quarter comebacks, 19 game-winning drives
Peyton Manning – 38 fourth quarter comebacks, 49 game-winning drives
Brady - 26 fourth quarter comebacks, 37 game-winning drives
Brees - 20 fourth quarter comebacks, 31 game-winning drives
Roethlisberger - 22 fourth quarter comebacks, 29 game-winning drives
Eli Manning - 24 fourth quarter comebacks, 28 game-winning drives
Rivers – 13 fourth quarter comebacks, 16 game-winning drives
Flacco - 10 fourth quarter comebacks, 15 game-winning drives
Schaub - 9 fourth quarter comebacks, 12 game-winning drives
Rodgers – 5 fourth quarter comebacks, 9 game-winning drives
Just for reference, after one year here are the GW drives for the rookies: Griffin (3), Luck (7), Kaepernick (2), Wilson (5)
Historical:
Marino - 36 fourth quarter comebacks, 51 game-winning drives
Favre – 30 fourth quarter comebacks, 46 game-winning drives
Elway - 35 fourth quarter comebacks, 46 game-winning drives
Favre – 30 fourth quarter comebacks, 45 game-winning drives
Montana – 31 fourth quarter comebacks, 33 game-winning drives
Staubach – 15 fourth quarter comebacks, 23 game-winning drives
Aikman - 16 fourth quarter comebacks, 21 game-winning drives
Steve Young – 14 fourth quarter comebacks, 17 game-winning drives
Danny White – 12 fourth quarter comebacks, 16 game-winning drives
http://www.pro-football-reference.com/leaders/gwd_career.htm
http://www.pro-football-reference.com/leaders/comebacks_career.htm
To put it in perspective, Romo shares spots with the likes of Neil O’Donnell and Trent Green in terms of career game winning drives and Jon Kitna in terms of comebacks.