There have been studies on elbow stresses by pitch. And I see people reading the wrong things in them. By most accounts a fastball puts more stress on the elbow than curveballs do. (Now keep reading). But the arm has the ability at different degrees to protect itself through pronation. Improperly thrown curveballs can severely reduce the amount of pronation as as the forearm and wrist can stay in supination longer, the pull down a window shade analogy. But what is the net of the combination of stress amount to pronation one versus the orher pitch? There has not been to my knowledge a credible study done on the combination of fastball and amount of pronation vs. curveball and amount of pronation to measure the net effect of stress on the elbow. So is there an answer, in my estimation, no not yet. But you can help prevent potential injury by understanding that a curve is thrown very much like a fastball just with a different forearm orientation and no wrist snap, so that the arm and wrist can go directly into pronation once the ball is released, don’t forcibly keep the arm locked in supination to manipulate spin.
ps. I am not suggesting pronation reduces the stress only that it protects against the effects of it. that is what i believe is missing in the studies