They are not efficient in terms of time spent. Active Isolated stretching or PNF are very rapid methods of increasing flexibility.
Basically, any method or device that stretches a muslce to the point of triggering the myotatic reflex and holds it there will be counterproductive if felxibility is the sole goal. Imagine dropping into a split right now, assuming you are not trained to do so. It will take between 2-3 seconds before your muscles tighten up and you feel pain, all courtesy of the myotatic reflex. Staying in that position beyond those 2-3 seconds will, by definition, be working against yourself since your muscles are tense.