What makes bits harsh?

Right on.

I actually hate rubber snaffles. Lol!

To the OP: look at the sticky JDI posted Bit Information (Snaffle and English-Type Bits) Lots of info including some discussion about the severity.

I have only ever seen one and I would never use one but some people think they're "kinder".

as others have said, the bit is only as harsh as the rider makes it. of course you want to use the softest bit possible (as close to a plain snaffle as possible in my opinion) to keep the horse from getting hard mouthed. my old mare needed a curb to listen because she had been ridden by so many kids, who always use the reins before thinking. where as Bali was brought up in nothing harsher than an aluminum snaffle with a small shank and I never have to even to touch him.