One of the big problems with using paintball guns and equipment for combat training is that a paintball can be stopped by thick brush, and induces people to mistake concealment for cover. A very fatal mistake.
Another serious problem is that paintball guns can be heard before the balls reach their target which encourages players to expose themselves until they hear shots fired. When you see two paintballers shooting at each other from a distance of 30 yards or more, invariably they will take turns popping their heads up and shooting until they hear their opponents gun firing back at them. In real life that would get you killed.