The flat bench press is a good movement, unfortunately 9 out 10 people ruin it by fucking up their form. Arching their back, bouncing it off their chest, etc.... It can be beneficial, but not the way they do it. Much like preacher curls. People go too heavy and their ass comes off the seat during the descent, then when they reach the bottom they sort of dip their ass and body downwards while dropping their shoulders and leveraging the weight back up. Everything they could do to fuck up the exercise they do and then decide it is one those exercises that "just doesn't work for them".