The last 4 QBs to beat the Patriots in the playoffs were Joe Flacco, Eli Manning, Mark Sanchez, and Joe Flacco. Andy Dalton fits right in with that group. It isn't good QBs that have hurt the Patriots, but good defenses, especially since this is their best secondary in several years. The Bengals smacked around the Patriots earlier this year. I'm not saying the Colts or the Chiefs are an obviously easier opponent, just that the Devil we know (Bengals) might in fact be worse than the Devil we don't.
FWIW I, In the TB/BB era in the playoffs, the Patriots are:*
6-3 if they beat the opponent in the regular season
3-4 if they lost to the opponent in the regular season
8-0 if they are playing a team for the first time (for those wondering, OAK 2001, PIT 2001, CAR 2003, PHI 2004, JAX 2005, SD 2006, JAX 2007, BAL 2011)
FWIW II, 2 of the 3 potential divisional opponents for the Broncos beat them this year, the Chargers (week 15 in Denver, 27 - 20) and Colts (week 7 in Indy, 39 - 33).
* I looked this up myself and wouldn't be surprised if there's an error in here...