Any bit relies on the horse knowing he can submit to pressure and the pressure will go away. Horses can be soft mouthed with any bit. On principle, I would say the best situation is whatever bit doesn't take muscle to work. I'd rather see a harder bit used lightly than a mild bit used harshly.
You are exactly right. You see people fighting a horse with a mild snaffle, all the while fussing about how harsh a "spade" bit is when the people that use them have fingertip control on an amazingly trained horse. I was just thinking about this the other day, funny that you brought it up.
I firmly believe that any bit is only as harsh as the hands that are using it. Except for those ridiculous bicycle chains I see people using. Sheesh.