In my opinion, a Dr Bristol is a harsher bit.
When this bit is in use, horses can learn to tuck their heads in to avoid the pressure of the bit, so in sense, the hores learns to evade the pressure from the pressure on the tongue that this particular mouth piece gives.
I remember seeing a horse at a HT with blood coming from his mouth. When the vet did an asessment, it was in result to the Dr Bristol mouth piece in the poor creatures mouth, from how harsh his riders hands were out on the CC course.
I dislike this mouthpiece very, very much.