Even when they won the Super Bowls I wouldn't call their defense "great". He certainly doesn't know how to build one, just look at the last 5 drafts as well as the last ten years of free agent signings he's made. Shanny made one good decision on defense in a decade and that was to get Champ. Dre Bly is a bust, the UT and Florida D-lineman have been busts, the linebacker corp for the last decade minus Al Wilson have been busts, the Browns castaways have been busts, and getting an old man to play safety year in and year out has been a bust.
Shanny doesn't know about defense and that was ultimately what led to his demise. His offense has been fine, although the play calling has sort of sucked ever since Kubiak left.