Even the points-allowed stat.
I believe the Cards have tended to give up a much higher-than-average points off turnovers than other NFL teams and that, at the very least, turnovers last year led to our defense having to do yeoman-like work defending our territory inside the 50 and keeping the opposition out of our end zone.
The problem with other stats (i.e. sacks, completion percentages etc.) is that Billy Davis (and Pendergast before him) have been masters at adjusting his coverages/schemes to cover up weaknesses.
Which means - If we can't get the job done rushing our front 3 (plus a LB), we have to rush more people thereby leaving our DB's exposed.
Or conversely, if our corners can't handle coverage on an island all alone, we might have to leave both safeties back to help out instead of moving one of them into the box to help stop the run or rush the passer.
Run defense stats are even more complicated because - if our front 3 does a good job of keeping blockers away from our LB's but the LB's fail to execute, the blame invariably falls on our D-line. Conversely, if our D-line isn't doing its job, it can make our LB's look really really bad.
That said - My own assessment is that (a) the front 3 of Dockett, Watson/Robinson and Campbell is one of the best in the NFL, (b) there was a lack of quality overall LB depth, no truly super-human athletes (especially outside) plus - Dansby (despite his occasional heroics) wasn't heroic in every game he played; and (c) we lacked a decent #2 cover corner and our FS who, though athletic, hadn't really mastered the mental/strategic part of the game.
Actually, few if any NFL defenses are 100% strong across the board - each have their position-weaknesses to varying degrees. But the more roster-holes you have, the more "unlucky" you're likely to be in some key games. I believe this is what happened to the Cardinals last season (& to a certain extent the season before).
Hopefully some of the roster-changes we made during the offseason will fill in some of the missing gaps, but we won't know that until "live" bullets start flying.