Behind-the-Neck Shoulder Press
Quick question: Would you jump off the roof of your house to activate the maximum number of motor units in your quads? I'm going to assume the answer is no.
Although you'd likely achieve more quad recruitment with roof-jumping than with anything you've ever done, it wouldn't really matter. You'd be too occupied with things like a) crying like a schoolgirl, b) calling 911, and c) looking for your kneecaps to be able to enjoy the awesome depth jump you just did.
Along the same lines, there's no doubt that behind-the-neck presses are good at stimulating the deltoids, specifically the anterior delts. But just because an exercise is good for your muscles doesn't necessarily mean it's good for your joints.
The main problem with behind-the-neck presses is that the movement has to be done with the shoulders in extreme external and horizontal abduction. In other words, you're required to do the movement at the very end range of motion for the shoulder joint.
Although it's normally safe to take your shoulder to its end range of motion if you're in the club "raising the roof," it becomes far less safe when you do rep after rep, set after set, with a loaded barbell in your hands.
While it's true that the shoulder joint (aka: glenohumeral) is the most mobile joint in the body, it's also the most unstable. So, just because you can actually get a barbell behind your head doesn't mean that you should do repeated movements, against a load no less, in that same position.
It's much, much safer to press overhead with the humerus moving in the scapular plane, which is about 30° forward of the frontal plane.
To find the scapular plane, raise your arms straight out to the sides (in the frontal plane) until they're parallel to the ground (as in the top position of a lateral raise). Now bring your arms forward about 30°. Your humerus is now in the plane of your scapula. This is the position your upper arms should be in when you do overhead presses.
Sure, there are some people who can do behind-the-neck barbell presses for years and never have a shoulder problem. Likewise, there are people who can smoke cigarettes for decades and never get lung cancer. But in both cases, you're gambling... and with odds that are not in your favor.