maybe a well schooled horse!
Any ideal candidate for a Meroth should carry their bits. If your horse doesn't pick up and pack their snaffle or whatever standard bit you have him in, then you're going to have issues with the Meroth staying put.
Horses with sensitive faces and soft mouths, I might actually try one for Bassira.
It would be ideal with a horse with facial nerve damage who cannot use a bridle but other than that it does not make much of a difference compared to another bit, unless the horse has an underlying bit confidence problem.
I've got one and I can tell you it totally changed my horse! I ride all the time with it now! My horse has a sensitive mouth, a simple bit doesn't correspond to him at all. I've changed for that kind of bit, and that's not the same horse, really concentrated, he wants to work now!
I have a meroth bit but I don't use it that much (just to swim) that's good because it's never damaged :) Otherwise I have to put a halter to fix it if not it doesn't stay in my horse's mouth :)