A broken mouthpiece has a snaffle action and IS less severe than a solid mouth piece
Why do you think that? A broken mouthpeice has a nutcracker action and can stab up into the roof of the mouth or down into the tongue. The sides also collapse on the bars of the mouth. Combining that with shanks drives the point of the broken mouthpeice down into the tongue as well as the bars collapsing on the sides of the mouth, as well as the increased pressure with all but the lightest aids creates a LOT of pressure on a lot of different places in the horses mouth.
A solid mouthpeice does not have a nutcracker action, and does not collapse onto the bars of the horses mouth. A ported mullen mouth, the more common type in curb bits (I think?) is even more mild. As the mouthpeice rotates in the horses mouth with the effect of the shanks, the port still allows the horse tongue relief. If the port is SUPER high, it can contact the roof of the mouth, but because it is solid it is still softer than the pointed end of a nutcrackered broken mouthpiece.
As said above, the mouthpiece of a bit has nothing to do with the distinction between curb and snaffle. The difference? A snaffle has a 1:1 pressure ratio. 1 ounce of pressure is put on the reins by the rider, and 1 ounce of pressure is felt by the mouth of the horse.
In a curb, this ratio starts at 1:2 and increases with a few factors, mainly the length and angle of the shanks. 1 ounce of pressure is put on the reins by the rider, and 2 or more times that pressure is felt my the mouth, chin, and poll of the horse.
That is the difference between a snaffle and a curb. Both snaffle AND curb can have broken, mullen, or ported mouthpieces.