NFL 3-4 defenses obviously need specific personnel. I think scouting these types of players is somewhat difficult. Hybrid outside linebackers in a 3-4 are hard to find. A scout must watch 4-3 rush ends on tape and in practice and make evaluations on overall athleticism. Scouts must simply make a prediction as to whether or not these players can make a successful position change at the next level. These defensive ends will not be asked to get out of a 3-point stance and stand farther back on the line of scrimmage very often.
There is enough evaluation involved when it comes to selecting collegiate players in the draft. Guessing about whether a position change is possible right off the bat makes things even more difficult. The draft is about acquiring talent that can help the organization sooner rather than later. The 3-4 automatically requires position changes to take place immediately.
Few college football programs run three-four defenses. Only a hand-full of players in the draft can play nose-tackle in the NFL 3-4 defense. A couple of defensive tackles will weigh-in around 300 pounds but have the sheer brute strength to at least be decent at the position. 350 pounds plus though is a general requirement. This position requires taking on at least two blockers every down, maintaining the line of scrimmage and causing as much inside havoc as possible to stop the run. These players are asked to do this for the first two downs of every series, then usually come out for third downs when a pass play is likely from the offense. Thus they are a one-dimensional player that rarely provides any sort of pass rush.
The inside linebackers in the 3-4 defense usually weigh-in at around 250 pounds because they are asked to take on offensive linemen much more often than their 4-3 counterparts. These middle linebackers though, are usually asked to have the athleticism to cover and provide a little pass rush just like the 4-3 middle linebacker. This type of player is also tougher to find in many peoples eyes. The 3-4 defensive end usually has as much strength as many 4-3 defensive tackles while being taller and having longer arms for taking on offensive tackles. Sometimes these players also played another position previously, usually defensive tackle in college.
Many NFL teams have recently switched their defensive scheme to the 3-4, but for many I think this will be unsuccessful overall. Why? Because the type of players they require are simply harder to find. Several teams will take players in the first or second round, expect them to change positions and learn the scheme instantly, but this will not be the case. Others will try to move NFL defensive players from the 4-3 around so that the 3-4 defense can be used. This will also only have mixed results.
Then why do I still favor a 3-4 defense? A successful one is dominant in many fashions. When the right players are added, both stopping the run and rushing the passer is made possible. When a dominant nose-tackle takes up two blocks and forces the runner outside, he has a huge impact on the game. Even if the nose tackle doesn't provide a pass rush, by taking up two blocks, he allows other players, probably an outside linebacker, to squeak through sometimes completely unblocked. If the front three have the strength to take up most of the offensive line, you have three to four linebackers running around free to stop plays. The biggest reason though that I like the 3-4 is the schematic possibilities. Linebackers can rush from many different places and leave offenses confused and exposed.