I've seen dozens of 'footers' that could ride all day long on a boom or bridle attached to the boom, but as soon as they get on a long line, they're good for about 5 seconds. Once the student can maintain good, no, perfect body position on the boom, they should be moved to a bridle attached to the boom. Once they can maintain the same good body position on the bridle, they should move immediatley to the long line. The end of the ride. Boom, bridle, or long line, at the end of the ride, make sure the student leans back.
Face plants ruin the whole experience.