It's been my experience when I see people break their shin bones it's because of improper training and execution.
The shin bone is more oval than round. So if you don't fully commit and execute the kick properly and hit with a slight angle and the impact is to the inside of the leg there is a greater chance of breakage. Think a ruler or a flat piece of wood. If you hit with the edge it's a lot sturdy. Hit with the flat side it's going to flex and break.
As far as instep we (thus I) do tsumasaki geri or toe kicks. They are effective towards flesh and sensitive areas. Allows for penetration into small openings within the opponents guard. Since I stopped with instep and ball I've had less injuries to my foot. Counter intuitive, I know. Also that option/concept is easily transferable to the street since a good boot, shoe, or sneaker won't allow me to pull my toes back anyway and the tips are very sturdy and effective as well.