Most people stop doing high frequency pull-ups because of an elbow injury. Sometimes the problem surfaces in the shoulder. In either case, the culprit is the same: the pull-up or (even more troublesome) the chin-up from a fixed bar.
When you pull in the vertical plane your wrists naturally want to rotate. How much they rotate depends on your skeletal structure and soft tissue mobility. Regardless, your wrists never want to be locked in place for this exercise. This is simple to verify. Work up to a 3-rep pull-up max from rings and watch what your wrists do – they'll never stay fully pronated on their own.
If the wrists can't naturally rotate, the stress goes straight to the elbow, leading to pain and inflammation. Then the shoulder will join the pain party. If you observe shoulder movement when a guy does a pull-up from a fixed bar it looks the same as when he does it from rings. However, there are small biomechanical changes when the wrists can't rotate. You might not be able to see a difference, but you'll eventually feel it when an underlying dysfunction rears its ugly head as shoulder pain.
The Next Best Thing
Now, if you can't get access to rings, TRX straps, or handles that rotate, the next best option is to do pull-ups with a neutral grip. The hammer grip (palms facing one another) is easiest on the elbows. Fattening the grip takes additional stress off the elbows.