Many coaches say the behind-the-neck pulldown is unsafe. It can be, if you don't know what you're doing. The problem is that lifters usually think of this as an exercise for the lats. It's not. It's a mid-back exercise.
How to Do It Right
Contract and squeeze the rhomboids and keep that position during the entire set.
Bring the bar down only to the back of the head, upper ear level. Do not bring it down to the neck.
Keep some bend in the elbows at the top. Don't let the bar pull your arms straight.
This is one case where a shorter range of motion is actually safer and more effective.