Power and comfort are increased by racquet stiffness. You will have more power with a stiffer racquet. When the ball is hit by a softer frame, it takes power away from the player. A stiffer frame absorbs more impact force, and the ball retains more of its power on impact.
Tennis elbow is less likely to occur in softer frames. The exact opposite might be true for other players. A-frame rate at a higher stiffness feels just as comfortable, if not more so than a frame with a lesser stiffness rating.
It has never been a substantial concern of mine because the type of tennis string you use affects the comfort more than the racquet stiffness. You should choose a less stiff racquet if you have arm problems or are worried about them. The majority of racquets have a RA rating between 55 and 75, with a lower number indicating more flexibility and a higher number indicating a stiffer racquet.