Any DC worth a **** is MULTIPLE.
Personally I like the 3-4 for 2 reasons...
1. It's flexible. It can look like a 4-3, a 4-4, a 5-2 or a 3-4.
2. It's symmetrical. Easy to adjust. You can flip your whole defensive play with one word or two words pre-snap and nobody has to move.
If I call "Odd" then we're in a base 3-4. DE's head-up on Tackles and Nose head-up on Center.
If I call "Over" or "Under" then we're in more of a 4-3 look, especially if my rushing OLB puts his hand in the ground.
Then I drop a Safety down and now we look like a 4-4.
3rd down, I take the Sam OLB and substitute him in at DE to rush the passer then bring in a 5th DB. Now we're in a 4-2-5.
IMO, it's because of this, that you need more versatile players to run it properly. I need, or prefer, OLB's that are equal in skill. I need Safeties that are equal in skill. Reason being, I can't always blitz the same OLB, I need to be able to change things up and send the other guy. I can't always drop the same Safety down into the box, I need to have the ability to drop the other guy down sometimes. This allows me to be more versatile. If I have one Safety who's a big, slow, physical guy and another Safety who's a ball hawk then I'm pretty sure the offense will figure out who I'm more likely to drop into the box.
Up front it'll always be attacking, one gap technique. Even when we're in a base 3-4 we'll be slanting one way or another. We'll also be pinching occasionally, i.e. both DE's slanting into B-gaps.
In the back-end we'd be running a lot of split field and combo coverages. Man/zone mixed. Quarters on one side, 2-read on the other. Man here, zone there. Yada yada yada.
It's not so much WHAT defense you run, it's the concepts you use. Saban's defense isn't as complex as people think. It's relatively simply and extremely sound. He minimizes voids and weaknesses by using innovative concepts like Rip/Liz Match. (his adaptation of cover-3 that guards the seams)