Some good stuff on behind the neck work:
"Concurrent extreme external rotation and abduction has been termed the "at-risk" (or 90/90) position by many practitioners, so behind-the-neck lat pulldowns and presses have been blacklisted. These individuals cite potential problems with anterior glenohumeral instability(4,5), external impingement, internal impingement (a new-age diagnosis common in overhead throwing athletes with hypermobility), acromioclavicular joint degeneration, and even the risk of intervertebral disc injuries (due to the flexed neck position). The infraspinatus and teres minor are shortened in the 90/90 position, and may therefore be ineffective as depressors of the humeral head due to shortcomings in terms of the length-tension relationship.(7)
In my opinion, you must view the two exercises independently of one another. For starters, one must differentiate between exercises involving traction and approximation at the glenohumeral joint. Pulldowns (like most cable exercises) are an example of a traction exercise, as they pull the head of the humerus away from the glenoid fossa. Various pressing exercises, on the other hand, involve approximation; they drive the head of the humerus further into the "socket." Approximation exercises increase the likelihood of subacromial impingement much more than traction exercises, and this is why exercises like pulldowns, pull-ups and shrugs can be integrated into rehabilitation programs before various presses. So, on paper, pulldowns in the "at-risk" position are less dangerous than presses.
However, in comparison to all the pulldown variations you can do in front of the neck, going BTN is just going to shorten your range of motion and reduce activation of the very musculature you
