I love what I refer to as the 425 Maim defense. Basically it's a standard 425 scheme, only the DBs walk up and mash the living crap out of the receivers.
But there are a few problems:
1) Without a lot of depth in the secondary, you simply cannot run this scheme even 20% of the time. It's too physically taxing unless you can roll in a new secondary every two or three plays.
2) Most DBs have never had to actually learn how to be physical, and to jump into a scheme that requires precision technique is extremely difficult.
3) I wouldn't be a very good offensive mind if I didn't have roughly 1,600 ways to counter a particular defensive alignment and scheme, let alone an entire defense predicated on a couple of very basic tenets.
Were I coaching against a team that ran this type of defense even under optimal conditions for them, it would take me roughly two offensive series to make the adjustments that would either run the defense out of the scheme or else get scorched for 70 points.
The toughest defense to go against is one that is extremely physical, extremely fast, extremely tenacious, and extremely deep. But even with a completely average defensive personnel group, a multiple look is much more difficult to counter from an offensive standpoint because setting up for a home run play is that much tougher. When the best that could reasonably be expected on a given play is maybe seven yards, the tendency is to scale back the offense into what would be a guaranteed 3-4 yard pickup a pop.
The problem with putting together such a defense is that the more time is needed in meetings and the film room, the less time can be spent on the practice field. This costs both reps against a live offense and also time spent reinforcing the techniques that make a defense successful.