NO has a brutal stretch of SF, @ATL on a Thursday and @ Seattle. Those three games likely decide who's the #1 seed. I don't see Seattle or NO going into the other stadium and winning a road game in the playoffs.
In the AFC the Broncos still haven't played anybody and while their offense is looking unstoppable their defense isn't great and the four teams they've beaten aren't good offenses.
As others have said all it takes is one injury to change the complexion of an entire team. The 2007 Patriots lost Colvin which moved Thomas and weakened the middle of their defense and then in the final game they lost Neal which really hurt the OLine especially coupled with a gimpy Brady. So it doesn't even have to be the stars. 2011 gimpy Gronkowski in the SB, etc.