It's only true that you can't overshoot them if your trajectory is perfectly vertical. With a diagonal trajectory (which is usually the case), you can still overshoot the item you were aiming for.

Also, Fitts' Law says that the time to select a target is proportional to the distance and the size of the target. Putting the menu on the edge of the screen makes the effective size of the targets bigger, but in some cases it also means that they are much further away.

As I type this, the menu bar is about 4x further away than the top of my browser window.