That's pretty interesting.. I'll have to look into this further.
Your shoulder issues may not just be because of tendonitis though. The shoulder is complex enough that often it's posture, biomechanics, limited mobility or imbalances that often screw things up there rather than strict tendinopathy in most cases.
Medial epicondylitis, and patellar tendinitis tend can be just overuse but are less likely in most cases to have lots of other things go wrong with them compared to the shoulders and for another example the low back.