Just a note - the slow twist and corkscrew are not necessarily harsh - they are harsher than a plain snaffle but some horses prefer either of these bits to a dr. Bristol depending on mouth conformation and tongue skin thickness. When you're talking details like this you're talking far beyond your average bitting, but I've also known horses to prefer a corkscrew to a slow twist and vice versa - all depending on the conformation of the mouth and individual horse.
My one TB jumper is an exception to a lot of bitting rules - hates a single jointed mouthpiece more than anything as he has a low soft palate and it hits the roof of his mouth and causes pain. He's very sensitive mouthed so even a three piece (of nearly any variety) he is resistant to. He prefers a mullen pelham - mullen b/c it's simple and pelham b/c he doesn't mind some leverage and this allows me to fine tune my cues to him (and in all reality the curb primarily acts as an e-brake since the mullen snaffle offers very little control OR communication due to it's simplicity).
I also have a horse that goes in a waterford full time - he's a rescue here that has been adopted and is boarded with us. He's another exception in that he had severe damage done to his tongue on the track and has massive scar tissue. In his case the waterford doesn't have the same jaw-breaker feel it can give other horses b/c his tongue's scar tissue is in the way. While normally a relatively severe bit, in his case it's one of the few bits he can actually FEEL and receive communication from, and he prefers it as otherwise he feels as if he doesn't hear/understand what the rider is saying.
These are just a few examples of horses I've seen over the years, and the uses for these bits. Every horse is an individual and a good rule of thumb to test a bit's harshness is to put it in the crook of your elbow, close your arm, and ask someone to "pull on the reins" which will in essence recreate the feel of the bit in the horses mouth - just imagine that it was much more sensitive tissue than the skin on your arm...
Always remember, what works for one horse may not work for another, although there are ALWAYS some bits imo that are to be avoided for riding at ANY cost such as the bike chain, regular chain, and double twisted offset wire.
ONCE I had a use for a double twisted wire and it was this: OTTB 4 years old, leaning heavily on the bit regardless of what you asked him to do. To test for nerve damage, put the twisted bit on him on the LONGE ONLY for a few minutes. Used side reins set too loose on purpose - not to encourage him down, only to act if he literally reached to the ground as he had a habit of leaning immensely. Well, as soon as he started to walk, and then same at the trot, he rooted (or so it appeared) and actually leaned ON the side reins and bit without being phased at all. We were pretty sure that he had nerve damage from whatever happened to him on the track, and called a chiro and a vet. Turns out he had a compressed vert in his neck that was reducing nerve sensations from that vert forward, which is why he would lean regardless of what you did, what bit, etc. He simply couldn't feel it. As for asking him to move forward and engage, he actually wouldn't respond to leg. We found out tragically a few months later that he had intestinal cancer (yes, at this point he was barely 5 years old) and literally lost feeling in his sides due to the tumors. It was the most bizarre case I've ever encountered but wanted to share as I found it interesting how in this case a bit I had only from confiscating it from a former client actually help start us on the track to diagnose the horse's multitude of medical issues.