I had a roping horse that I could only use a smooth chain bit on...he had been handled with a heavy hand prior to me getting him and ANY kind of bit other than the chain would make him jerk his head up as soon as you picked up the reins on him--NOT pull on the reins but just pick them up. With a smooth chain he wouldn't jerk his head up(I am sure he would if you were heavy handed). If you put any other kind of bit(including a ring snaffle) on him he was instantly apprehensive and "looking" for you to pull the reins.
This horse would stop on a dime with just you sitting down in the seat, so there was no reason to ever do more than slightly bump the reins....this leads me to believe, as others have said, that a smooth chain is not that harsh, but could be in the wrong hands-as all bits could.