I've seen the bashing on chain mouthpieces in both bit threads and I thought I would share my experiences.
I have a wonderful 9yr old AQHA mare, Friska who is the best lesson horse anyone could ever hope for, she has the most forgiving personality of any horse I have ever met. She rides in a chain mouthpiece jr cowhorse bit.
I got her as a late two year old (one month from turning three) and she was lightly started in a sidepull before I had an accident with another horse and broke my femur, at the time I was 15. 8 months later I was back in the saddle and she was still in a sidepull. As a 4 year old I finally decided I needed to teach her to ride with a bit. My sweet little mare who could be trusted to pack around my 5 and 3 year old cousins without ever putting a foot wrong turned into a monster when I bridled her. Yes her teeth were done.
She took the bridle fine but when I stepped back she would try and kill herself. I tried all of your go to bits to start horses in, I tried every cheek piece, loose ring, eggbutt, d ring, full cheeks and bauchers. I tried single joints, french links, mullen mouths, rubber mouthpieces, thick mouthpieces and thin.
What did she do you may ask? She would flip over backwards, rear up and paw at her head, turn and itch her head with a back hoof, itch on fences etc. She had more self mutilated head wounds than any horse i've ever seen. She absolutely panicked once I was out of the way. For a long time I thought she would never ride in a bit. It was a trick to get in there and rescue her when she got herself hung up.
Now I was young and stupid and just blindly trying bits without really thinking about it. It wasn't until I found her current bit, the chain mouthpiece jr cowhorse that I started to figure things out. When I did bridle her with that bit, there was none of that self destructive action. She mouthed it for a few moments and that was it, I had a happy horse. Now I ask you, if it was really that horrible of a mouthpiece would a horse go from being so self destructive to content?
Friska has a very very fat large tongue and a low palate. There isn't really much room for a bit in there. The chain mouthpiece is flat and curves around that massive tongue of hers so it doesn't get in the way.
A well made chain mouthpiece should have no rough edges. All of the links should be smooth. Yes, if a rider had abusive hands and see sawed at the bit it would rub but that isn't how any bit should be ridden in. If you have abusive hands and haul back, yes it will wrap around the horses mouth. What bit isn't harsh if you ride in it that way. These bits are meant to be used on a loose rein, they aren't meant for contact work, they're just too bendy, that doesn't make it harsh.
Friska is a lesson horse and she is regularly ridden by children and adults with uneducated hands that do all sorts of bad things. Never does she pin her ears, pull the reins out of a persons hands or even duck behind the vertical to evade the bit.
For those wondering if i've tried this bit, yes I have. I've had it in the crook of my elbow, the back of my knee, and between my fingers to test how it feels. Even with harsh hands, unless you really see saw or yank back it is quite comfortable.