A good example is moving the camera: lets say you have a camera set to follow a particular character (in some fancy way), but you only update it in FixedUpdate(

A good example is moving the camera: lets say you have a camera set to follow a particular character (in some fancy way), but you only update it in FixedUpdate(). If your framerate is faster than the physics update, FixedUpdate() will not be called every frame, and your camera will get a visible stutter (it will essentially be "temporally aliased" with the physics updates). Therefore, camera movement updates should always go in Update(). This kind of reasoning applies for many "game logic" things. There's a flip-side to that though: if the stuff in the world are physics objects (especially if it's your main character), and they only update in FixedUpdate(), then even if the camera moves smoothly, the objects will stutter. Hence, you need to do some kind of interpolation or something of them as well in FixedUpdate(). The point is: this stuff is really, really hard to get right. There is no simple right answer.