Stunts and blitzes make a good pass defense better, and a bad pass defense worse. Stunts take long to develop leave gaps seams and open routes, Dropping a lineman into coverage is meant to confuse a hurried quarterback. If you cannot get a rush with 4 or 5 down lineman, you certainly aren't going to get one with 3 or 4. Those are personnel problems. Coaching problems are very simple. Confidence in diagnosing a gameplan. A stunt twist or blitz the wrong way at the wrong time results in a huge play, its a gamble. Some coaches are confident in their ability to study film and gamble accordingly. Coaches without confidence auto-demote themselves to a "bend but dont break" philosophy, vanilla defenses that tackle pretty well, and they hope to hold teams to field goals using a shortened field as a cheat sheet for calling aggressive defensive plays. Usually this kind of philosophy only works with an offense that can run up the score. Opposing offenses will start making mistakes to try and catch up. See Oregon ducks.