Advantages
Madagascar can be used as an outpost, though it does not have the same strategic impact as outposts in Iceland or Alaska.
The large expanse of the Saharan desert gives extra time for detection and destruction of incoming warheads.
Disadvantages
Africa's major city, Cairo, is extremely close to enemy lines.
Africa's centralized location causes it to be a major target, and defense can be difficult due to attack coming from, literally, all sides.
Either/or
Africa's close proximity to other countries may cause incoming hostile warheads to be shot down by other defence networks, though this can also work to a disadvantage due to the additional defence Africa's own warheads must pass.