Revolut doesn't cap ATM withdrawals but it charges 2% for amounts above that in any given month. That might still work out cheaper than exchanging cash, but it would obviously depend on the availability of USD- dispensing ATMs in whichever African country you are going to, and on the additional fees these local banks might charge. Alternatively, there are some USD-dispensing ATMs in Dublin (I have seen them at the airport) but my hunch is that they might have layers of surcharges.