With football, you can make a reasonable approximation of how players will perform at the pro level as opposed to how they did in college. Granted, "sure things" and "can't misses" flop all the time, and guys come out of nowhere to do well.
But drafting high school and even college kids, and trying to project how they will handle big league pitching, or whether pitchers will be able to get big league hitters out, is a total crapshoot.
The thing that makes NFL and MLB projections totally different is the basic difference in the games. There is nothing that compares to the individual confrontation between pitcher and hitter. That is the toughest thing to project.