"The Broncos are ranked first in total defense, giving up just 258.5 yards per game. They rank second in rush defense (71.8 yards per game) and sixth in pass defense (186.7)."

What's for you to buy? Numbers don't lie, all you have to do is actually watch the games, the D is more than fine.

They will give up some plays, that'll happen when the Offense goes three and out all the time. They get short fields from horrible ints    and some of the O's possessions are 58 seconds long, think about that. After a bit, the D might get, you know, tired.