I put one of my mares in an MB43LP which is the low port version of the D-ring with a ported mouthpiece and hooks. She was a pretty downhill, heavy mare who wanted to lean on the bit all of the time and she went much, much better in that Myler than anything else I ever rode her in. I did use the hooks so that the bit had a sort of modified or muddled gag action, and then I added a curb chain when we went to shows to make it easy to pick her up in front of the jumps.
I ride with Greg Best a lot, and he was less than a fan of that bit. So I tried several other types of bits with her (after showing in the Myler for 4 or 5 years). But I never found anything I liked anywhere near as much, so she still goes in her Myler, and I’ve had 10 great years with her including several years in the 1.30m and 1.40m classes and small GPs.
I also had a very upright Dutch horse (his neck came out of his shoulders pretty much straight up), who was really fussy with bits and wanted to invert before accepting contact all of the time. I rode him in a loose ring fat snaffle for years before my trainer at the time suggested the myler (which is the same one I ended up later using on my mare from above). It was the first time that horse ever softly accepted contact without a head toss first. I rode him in it until I sold him.
I haven’t had as much luck with my current horses. My TB wants to curl a little bit too much with my MB43, and the comfort snaffle I have isn’t enough to keep him together. My young horse was too green for my MB43LP when I was experimenting with bits for him and I couldn’t find my comfort snaffle when he was young, so he goes in a french link full cheek that I’m happy enough with for now.
I don’t think Myler bits are magical or right for every horse, but when I have a horse that’s a little tricky, it’s usually the first thing I reach for.