You also have to keep in mind that the type of 3-4 defense matters a lot. The "classic" 3-4 is also called a 2-gap 3-4, because the 3 down linemen all have to cover 2 different gap assignments - this is why they typically don't have huge sack numbers. In a 2-gap 3-4, their main job is to maintain gap responsibility and cover the run.
However, there is another type of 3-4 called the 1-gap 3-4, which has the 3 down linemen play a more penetrating style, responsible for attacking a single gap. This is the defense that Bruce Smith played in (and the same one that Dallas runs as well with Wade Phillips). Here, the 3-4 RDE plays a role similar to a penetrating 3-technique DT (a-la Warren Sapp), and can, if you have the proper athlete, get consistent double digit sack numbers. This is also why Dallas can get away with Jay Ratliff as their NT, since instead of being responsible for 2-gaps, he's responsible for penetrating the A-gap.