Any bit is harsh if used strongly. Single jointed snaffles are no exception & definitely not soft or even 'softest'. If used with any force, 2 reins at a time, they can gouge the roof of the mouth and act like a nutcracker on the tongue.
I have no problem with bits, if they are used gently for precise communication, rather than control on a horse who lacks enough training to respond without force. If the horse has been trained well prior to bitting and the rider has 'soft' hands, then (almost) any bit can be considered 'humane' in most situations.