That's because often enough there just isn't a way to adjust them properly. To get cantilevers working right all the parts - frame, brake mechanicals, pads and rims - all have to work well together. You have to try different combinations of these components until you get a full set that works together, otherwise all the adjustment in the world isn't going to help. If you have money and time and a pile of spare parts to throw at making them work, that might not be a problem. Or maybe you'll just get lucky. If not, they're just an eternal pain in the bum.