It all depends on your flexibility, past injuries & posture. Can you lower the bar down comfortably full R.O.M. WITHOUT pain, or bending your head downward i.e. keep looking straight ahead or sticking your head forward like some deformed chicken???
Pressing behind the head is safer than pulling down, same rules apply - but think... there is not 1 sport or job that requires you to pull a weight down behind your head in a seated position....
NOTHING should move in this exercise except the lats, shoulder girdle & arms.
If pulling to the front, NO LEANING BACK of your torso - turns it into a half-assed horizontal row that overloads your rotator cuff.
So behind the head pulldowns are an unecessary risk for most, but they can be done if you meet the above criteria and the boom of the pulldown is long enough to extend behind your head so as to allow the bar to drop straight down.
S