Skills, and taking 2 years off stunts the development of those specific skills. Quarterbacks are not working on throwing motion and footwork or studying defenses. Wide receivers are not catching 200+ balls a day or running routes. Basketball players are not working on their handles or their low post moves. Yes, they eventually get better than they were in HS, but it takes a lot of work just to get back where they were pre-mission.

Coaches have the option to redshirt a player, and help him develop more and many take advantage. But these players get to be in the weight room, studying film, and practicing with the team.

Also, for every player you get coming back that works hard to get back, you have another that lost a lot of the fire that made him so great before he left and he never excels like he may have of he had never left.

Are there pros? Yes. But just about every coach in the nation would advise and encourage the recruit to NOT go on mission because the pros do not outweigh the cons most of the time.