All that I have seen are based on timing. You press the button when the ball is released and let it go when the ball hits the glove. Then based on the distance you tell it the ball traveled, it comes up with a speed.
Issues? How accurate are you on your button pushing, are you pushing too late, releasing too soon? And how far is the ball actually traveling, from pitchers hand to catchers glove? You end up with an average speed, not the fastest speed of the ball like most guns, and not the speed at the plate like the catchers glove dohickey.