I've used grow bags for potatoes with mixed success. The first year was great growth above soil but not below soil. Potatoes were small and I had springhoppers or some kind of pest in the soil which ate most of the potatoes. Tossed that soil completely. The 2nd year, I had less growth on top and bigger potatoes but fewer of them. The 3rd year, my potato starts got about half way along and all died. Not sure why....
This year I've decided to not use the grow bags at all... I'm going to try my potato starts in tires instead.