In a bit, harshness is like beauty...it generally lies in the eye of the beholder.
(And, let's have clean terminology: snaffle bits do not have leverage; any bit with leverage is a curb.)
Sometimes, though, it's quite apparent. Bicycle chain mouthpieces, thin wire mouthpieces, chain-saw chain mouthpieces (yes, I've seen them used on mules). mouthpieces with intentionally roughed surfaces (think a farrier's rasp), etc. are examples of clearly harsh items.
The longer the shank on a curb bit the more powerful, and potentially painful for the horse, the bit is.
But myths prevail. "Thick is not harsh." Well, if it's so thick that it's too big for the horse's mouth then it's harsh. Conversely, thin is not harsh if it properly fits the horse's mouth. And in either case it's the hand that supplies the active power. So either can be harsh or soft depending upon how it's used.
The bit is a communication device. What do want to communicate? With a youngster you want to communicate basics so you ran use a bit that doesn't do much more that say "gee" or "haw" or "whoa." As the horse gains skill you can use a bit that allows more nuanced communication. When you get to upper levels of disciplines then you can see things like the double bridle or spade bit used effectively (and humanely).
Put another way, you can communicate with your horse via tin can telephone or high quality digital phone; which do you prefer?