Growing up in prep school/juniors I saw plenty of AC joint injuries and fractured clavicles in guys that were wearing those style shoulder pads. I had a separated shoulder myself wearing Douglas Defenders.
With the design of those pads, the soft padding was really thin under the plastic shoulder cap at the point of the shoulder because that’s where multiple layers came together. So when you hit the point of the shoulder most of the protection was the plastic cap, not the soft material, meaning it didn’t absorb as much energy. I saw fewer shoulder issues with guys that we wearing heavily padded shoulder caps - like the old Cooper Techniflex or some of the Jofa’s that were out around that time.