Any piece of tack can cause pain if used wrong. If you cinch up a horse too tight. If you use an ill fitting saddle. If the saddle pad wasn't inspected and has a burr. The throat latch is put on too tight. The list goes on and on. I think the best method is to try and see what works with your horse the best. If they hate and react poorly to a piece of tack, try something else.
I have a old, old gelding. He had been abused horridly in his younger years. Someone misused a bit in his mouth and almost cut his tongue off. His tongue healed, but it isn't straight because of the horrid scar running across it. However, he doesn't mind packing a bit around when used gently.
I have another gelding that was always ridden in a running martingale, because he would pull and flip his head with a slip ring snaffle. I switched him to a Kimerwick bit with a dog bone in the middle of the snaffle. He is no longer in a martingale, he doesn't flip his head, and has learned that he can be ridden in a bosal and not try to fight it either. He had been so use to hating the snaffle that he tried to get away from it, to the point he was dangerous in the bosal. He hid me in the head as we went up a hill the first time I rode him in the bosal, broke my sunglasses across my nose.
My filly I am training to drive now, hates a bit. She hasn't worn one much, but so far is really against it. For the time being I am ground driving her in her halter with no issues. I may go bit less for her.
I think people have this tendency to get into a mindset and not look with logic at the either side. Read your horse, understand your horse. Do what works well for your horse. Bits can be cruel to a horse that hates them. Teach them to "hold" the bit and you can go far with such light pressure that there is no way it can be cruel unless you chose for it to be.
Good luck to everyone, and listen to your horse. They will tell what is right and wrong for them.