I think I can put my finger on one reason I have trouble evaluating linebackers from film. The films on linebackers always concentrate on the guy lining up the running back and making the big hit. It makes for an impressive showcase for a linebacker to show him playing aggressively and making a hit and tackle. The problem is, the films don't really show how linebackers have to play.
The typical linebacker film has the linebacker playing in the middle of the field with the ball going to the running back. The linebacker waits for an opening and then shows a burst of speed to get to the back, standing him up and bringing him down with a thud. Nice.
What does a linebacker in college have to do? Rarely does he get the opportunity to lay a guy out like you see on the films. His first job is just to pursue the ball carrier and get him to the ground. Big hit usually not necessary. The films do an OK job of showing pursuit-and-tackling. Or at least, they usually do. Usually you can see how a linebacker pursues and whether or not he's a sure tackler when he gets where he's going.
After that is where the films fail. They almost never show the linebacker performing his second task, which is to drop back into pass coverage and defend the running back, tight end, or over-the-middle receiver. You just never see it, and it's such a big part of a linebacker's game in college, especially if the linebacker wants to play every down. If you see it, it's because he managed to get an interception, or a big hit after allowing the catch. Interceptions are nice, but I'd rather see five plays of a linebacker sticking like glue to a running back and not giving the QB the option to throw at him.
Evaluating a linebacker without seeing his coverage skills is like evaluating a baseball pitcher without seeing his curveball. How could you even begin? How much confidence could you have in whatever conclusion you drew?
The third most important part of a linebacker's game is to be able to shed blocks and still make a play. Film rarely shows a linebacker actually being blocked, I suppose it's because those kinds of plays aren't very spectacular. You can't usually get a big hit when you're just trying to get a lineman off of you so you can get any kind of hit. You almost never see this.
To continue the analogy, now you're evaluating your pitcher without seeing a curve or his 3rd pitch.*
There are other aspects of being a linebacker, such as being a situational blitzer. The films often show a linebacker blitzing and mauling the quarterback, so that's all good, but this is done primarily on pure athletic ability. So much of the linebacker game is mental, but the films focus on showing the athlete. This is what makes it so hard to differentiate the films of one very good athlete from the films of the three or four other very good athletes you're watching.
The good news is that the coaches know a lot more than we simple observers know. They see these guys play actual games rather than select plays, and can see everything these guys can do. They can make better evaluations than we can.
It looks like LSU will be picking up a commitment from a linebacker out of Florida today. His film is like all the others, but we have on pretty good authority that he is the top rated out-of-state linebacker left on LSU's board, so we'll be very happy to have him.
*Analogy does not apply to knuckleball pitchers.