They are harder, exactly because they're different. With mouse, you control how your crosshair moves by providing the game with starting and ending positions and coordinates inbetween. With controller, you control crosshair movement by sending how fast it should move and in which direction. First approach allows you to get pinpoint precision, while second is not and will never be able to. It can be compared to running vs driving respectively. While running you can stop very close to a certain line and not cross it. But when driving - theoretically you can do the same, but practically most of the time you'll be either overshooting the line or stopping to early.
