With their long history of use in live broadcast AR, mechanical tracking systems are still considered to be dependable and are the mainstay of many sports and live events, especially in larger arenas.
However, mechanical systems can require significant setup time and they require regular realignment. It is also necessary for the camera rig to have a fixed origin from which the movement of the mechanism can be referenced, limiting flexibility.
Optical camera tracking systems, on the other hand, can give far more freedom to move the camera around. Cameras with optical tracking can be moved freely; you can even use a steadicam.
As the tracking sensor typically points upwards or downward rather than into the set, it is unaffected by studio conditions such as lighting configurations, reflections, and plain green screens. Setup can also be relatively quick and easy.