Problem is adjusting the shoulder belt in an unsafe manner in order to ride in comfort.
It is not safe to adjust your shoulder belt inappropriately in order to enhance comfort. Wearing it too loose, under the arm or behind your back is not wearing the belts properly. Proper is how the auto manufacturers suggest-firm against your chest and tight on your lap.
Wearing the shoulder belt firmly against your chest means there is less gap between yourself and the belt. A gap causes delay in receiving a crash signal so your head could hit the airbag quicker and deeper. Firm against your chest means crash signal activation is more immediate so your head will not hit the airbag as fast = seconds count in a crash.
Rubbing by the shoulder belt is annoying for some but for people with chemotherapy ports, chest surgery, shoulder surgery and some shunts it is intolerable. One solution is to move it out of the way and another is to ask the Doctor for an exemption letter.