What makes a bit severe is ignorance, poor training and fear. There is no other answer to this question. In some hands a twisted wire snaffle is a refining tool and in other hands a rubber snaffle is a torture device. You need to educate yourself and your horse to overcome the ignorance and poor training and learn the proper application to overcome the fear and prejudice.
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.