Trainers are not socks, one size does not fit all. Trainers can help fighters if the fighter will listen. Some trainers can be the wrong choice for a fighter, too. You can't remake a fighter into the image of another fighter. Trainers can make improvements here and there but no one is going to to be training a batch of Mayweathers into existence. The best trainers know their fighters faults and strengths and try to suppress the faults and strengthen the other. I like Steward and Roach because they give the best advice during the fight.