Some (like my Foxtrotter Loki) are great in a simple snaffle bit or a shank bit.
Others, like the MFT I rode the other day, worked best in an extremely mild shank bit. It had a slight curve in the solid mouthpiece, and nice short shanks. He was extremely light and soft to ride, a real pleasure. Very well trained.
If he was in a snaffle, he most likely would have not been. He did best with seat and leg, light contact in the mouth. With the directness of the snaffle, he would have been nothing but a problem. Head tossing, rearing, etc. You would need to apply much more rein to achieve the same results as what you got with the shank bit.
With him, you deepened your seat and suggested with the rein and he would stop or (if asked) back up. A minuscule amount of rein contact was all that was needed, like I said, he was very soft and responsive.
He liked to have his head, carried himself beautifully, and neck reined very well. He liked the bit and so worked well in it. My TWHx Sammy hated shanked bits. He liked his snaffles or hackamores, or even better, just a halter.
It varies from horse to horse. Some will completely ignore the snaffle and be hard-mouthed beasts in them, but will be soft and light in a mild shank. Or vice versa, hard in a shank, easy in a snaffle. Some work best in a bitless or hackamore. Just depends!