This is going to be a long post and I apologize in advance. It will be full of personal experience and opinion and no scientific fact.
When I got my horse I borrowed some tack from a friend to see if he was trained to ride. As I approached him with the bridle equipped with snaffle bit he became anxious. He pawed the ground and lifted his head out of reach. He was extraordinarily unhappy. My mom was helping me at the time since he lives on her property. She only ever rode horses in mechanical hackamores the last time she had horses. She thought bits were cruel. Couple her beliefs with my horses reaction and I started to think that maybe there was something to that. I knew my horse had some sore teeth so we decided to try a mechanical hack. I approached the horse and he calmly accepted the hackamore. So ok, this would be what we were going to use.
And we used it for nearly a year, he rode ok in it (even put it on by himself lol) but he is a stubborn horse and I had a hard time giving him subtle cues as well as *making* him listen when he was having a bad day. My mechanical hackamore had 8" shanks. So my understanding is that they amplified my force by 8x. I am not a strong person. I'm fat, short and have very little upper body muscle. I welcomed the extra 8x force. Because let me tell you, if that horse did not want to turn there wasn't a darn thing I could do short of applying EVERY OUNCE OF STRENGTH I POSSESSED on the reins. Now he had good days and bad days but the bad days were more like horse wrestling than horse riding. So I knew I wanted to give a snaffle bit another shot. Afterall, that's what all the people on horse forum swear by.
I bought a 5 3/4" loose ring snaffle bit from a tack swap for $5. I have a brand new horse. He holds his head still and does not toss it. I can give subtle cues and he responds. If he is having a bad day and being stubborn I can calmly direct rein him to turn in the direction I want and he gives instantly to the bit. I can use a minimal amount of strength.
It didn't take me long to figure out that for now our best bet is to stick with the snaffle. He seems happier, possibly because we are able to communicate more easily and we are no longer fighting during our rides. If he pitches a fit about something I am easily able to control the situation and turn it around. Maybe it has made him more confident in me as a leader? Maybe he's happy to not have 8x my force pulling on his nose. I don't know, but I'll take what I can get and I truly believe that calm, quiet communication is certainly less cruel than wrestling and frustration. And that goes for any type of bit, hackamore or bitless bridle. There will always be different solutions for different horses. A style of head gear is not inherently bad or good.
Here is my old man on Christmas showing off his bit (our second ride with it).