When evaluating either side of the ball, you really have to look at the big picture; coaches, player development, draft evaluation, injuries, etc. I think coaching is too easy of a target and can't be the only thing looked at.
Unfortunately, IMO our draft evaluation on the defensive side has been average or below for quite awhile. We have suffered more busts then booms on that side of the ball (especially with high picks). Some of this can then be put on player development and coaching of these players once drafted. But I think when you evaluate how well we do with UDFA's and late round picks, our development and coaching does pretty well.
A caveat to all this is.......how do other teams fair with draft evaluation? Probably similar would be my guess. We have really hit the mark on a number of offensive players drafted in the last 10 years, which is why to many people it feels like we aren't hitting the mark in the draft on defense?