Sanding of any kind is "gentler" than end mills.
Particularly with antler, casein, bone and other natural products (less so with wood), an end mill can leave small "chip-outs" that can lead to cracks later.
The sanding process is much "gentler" to the material, and if it is done when the blank is near finished size, it also can be very accurate (you can SEE the tubes and just "kiss" them--making your length better).
FWIW