This plot shows why a single African currency under a supranational bank will be nearly impossible to improve the welfare of Africans. There is no banker in the world that can design a welfare-boosting monetary policy in such a heterogeneous market as shown in the image. By looking at the export nexus, what Nigeria needs is orthogonal to the needs of Rwanda, and yet you want them to be driven by a common monetary policy. Scale that to 30 divergent shocks, you have multiple-whammy paralyses.
One of the biggest challenges the monetary union which African Union plans to implement in the continent will be lack of flexibility to use monetary policies to drive economic agenda for respective member states. In other words, if a supranational bank which will become the central bank of the member states takes over, monetary policy tools cannot be deployed to fix some economic situations at member country level.