Emmortal offers an interesting quote. Reminds me of the first year in college when the prof told us about the major dangers of sneezing , with the high amount of negative internal pressure and potential rupture of blood vessels it put upon the body. Not to mention serious brain damage due to sudden head/neck jerking motion. Bottom line, never sneeze or serious injury may follow. Do not know if certain sections of graveyards have secret plots for victims of sneezing or not.
Emmortal quote does suggest a text book look at exercises preformed behind the neck. That being said, most any exercise will have a disadvantage for some folks.Not all exercises are meant for everyone. The Dl can be a prime example (could add squats and benches to that). Some should avoid it at all cost and others thrive on it.
As with the PBN, for people who are suited for it, it can add extra width and thickness to the whole shoulder girdle. And even strengthen that area quite well. A lot depends of each of our body leverage, joint placement, tendons, etc. Once a point of good flex/stretch is gained, if trouble or discomfort happens when doing the movement, than that exercise is not for you. Good Luck.
Side Bar: seen a couple of guy's do jerk press behind the neck and others do set's of 3's with very heavy weight. Not for everyone of course, but for them excellent results follow.