Any bit can be harsh in the wrong hands, I wouldn't say a curb chain is necessarily harsh it's just a different sort of action. Bits can work on various parts of the horses mouth and head, the poll, the bars, the tongue, the chin ect. The severity of a bit often predominantly depends on how how heavy handed you are and not upon where it acts.
A simple eggbutt snaffle can be considered harsh with a heavy handed rider. Hope that helps!
