All bits can* cause pain ! I have definitely caused my horse pain in a single jointed When he spun around from spooking and I bopped him in the mouth so hard (didn't mean to) he had his mouth open for a couple of seconds. When he bolted for the first time and I had to pull back (pressure and release) and see saw with all my weight for him to stop, I had no idea how to stop him other than pull back because Whoa wouldn't work and I didn't have much experience cantering in the paddock behind his best friend :/. A bit is a training tool that can* causes pain, or allot of pressure depending on how you use the tool.
The foolish reject what they see, not what they think,
The wise reject what they think, not what they see.