Sometimes armside run comes from opening early in addition to the arm slot. That can cause arm problems.
Just anecdotal evidence but I've generally seen that a lot of the kids who threw hard early seem to run into arm issues by the time they are about college age.
As far as fastballs being harder on the arm, people base that on recent info from ASMI showing that properly thrown curves stress the arm less than fastballs. However, in the past ASMI has done studies showing that throwing a curve increased the probability of elbow and shoulder pain, especially shoulder pain. My elbow always hurt after throwing a lot of curves so the part about the shoulder surprised me. The fact that kids end up with more injuries when they throw curves trumps the study saying that there isn't more stress from throwing a curve properly by a longshot. Guess what? The properly thrown curves don't work as well. Guess what? The kids are going to figure out what works best and throw it that way no matter what you tell them.