I'm not saying it is not a good exercise. It can be.
The problem is that, as I mentioned, few people have the capacity (strength and/or mobility) to do it properly and effectively. And if I just go out and say that it's a great movement then people who have no business using it will start to include this movement.
From a strength perspective you are about 30-50% weaker on behind the neck pull-ups than regular pull-ups.
One should not use behind the neck pull-ups as a training movement if they can't get at least 6-8 solid, controlled and full range repetitions on the movement. From a strength perspective that means that a 180lbs individual would have to be able to do 6-8 solid reps with AT LEAST an additional 50-60lbs on WIDE pull-ups from the front .... that is the minimal strength level needed to even consider doing the behind the neck pull-ups as a training movement.
But there is also the issue of mobility. I see a lot of people who have to round their thoracic and cervical spine to be able to "do" behind the neck lat pulldown. These people have no business doing behind the neck pull-ups even if they have the proper strength level. IMHO mobility-wise you need to be able to do he following with solid form if you are to even attempt a behind the neck pull-up:
- Behind he neck lat pulldown with upright torso
- Behind the neck shoulder press with the grip you would use on behind the neck pull-ups
- Overhead squat with the grip you would use on behind the neck pull-ups
If one has for objective to work up to using behind the neck pull-ups in their training I would suggest the following strength progression:
Work up to 10-12 strict chin-ups (supinated grip) with a 1 sec pause in the bottom position and a 1 sec hold with the chin above the bar on every rep.
Work up to being able to do 3 sets of 6-8 reps on strict chin-ups (same form as above) with an additional load of 30% of your body weight
Work up to being able to do 3 sets of 6-8 reps on strict pull-ups (shoulder width pronated grip) with same form as above
Work up to being able to do 3 sets of 6-8 reps on strict pull-ups (wide pronated grip) with same form as above
When you can do that you are ready to include behind the neck pull-ups in your training (strength-wise).
While you are going through the progression you should also work on the 3 exercises I mentioned above to ensure proper shoulder/thoracic mobility to do the behind the neck pull-ups.