I personally perform this exercise both ways. I actually do pull-ups the same way (in front and behind my neck). I have trained clients who could performed lat pull-downs behind their neck and if I see they can do them with no issues or problems, then I let them. Most people have hard time executing this exercise behind their neck, but there are few out there who can, it depends on the person. I’m sure each trainer has his/her own opinion on this, but mine is neutral.