I find out people who complain about the low bar to have a weak low back relatively to their legs. Thus the "high stress to the low back" feeling takes place. Also, they might as well be keeping a more vertical torso position, instead of leaning forward and focusing on the hip drive.
Based on Physics, the farther the weight is on a stick, which is being held on the other end, the highier the tension on it, the more it tends to bend over, considering the same weight always.
Taking this to the squat, the highier the bar position (considering the same torso angle), the highier the stress on the low back.
I'd appreciate any thoughts or corrections though..