It's the lack of eccentric/concentric transition... I once did an experiment with the bench press. Training it from pins only, using various heights... my bench press from pins increased a lot, up to 425lbs from the bottom position. But my actual bench press went down from 425 to 365!!!
I'm not saying that they don't have a place, but it's far from being a magic bullet even though the theory sounds tempting. Do not forget that Anderson was a natural squatter with perfect squatting levers.