Malaria would stil be a problem--as it is in Rome at this time. So would Trypanosomiasis (Sleeping Sickness) since it is spread by Tsetse Flies and also affects cattle. If a Roman navigator got as far as say, Angola (then settled only by San! (Bushmen), there would be fewer people to trade with but disease might not be such a big problem. But the Benguela Current would be challenging and might well defeat navigation to Namibia and the cape, considering that thee are few places to suppply water for rowers in the Namb Desert. Nothing along the entire Skeleton Coast until one coms to Walvis Bay and then nothing until the mouth of the Orange River and then nothing until Saldanha Bay and the Cape--which would also be extremely challenging for a Roman trireme.
Remember, Hanno did his feat going the other direction and had the Agulhas and later the Benguela Currents with him the entire way. Any Roman navigator would likely need to do this from East to West. It would liteerally be a one way tade route given the seafaring technology the Romans had.
But strangely enough, considering the difficuties of navigating the Red Sea (prevailing crosseinds, few places to water rowers, hostile states on both the Yemenite side and the Somali side, once discovered, because of a combination of favorable currents and winds most of the way and trading opportunities in West Africa, the route around the cape to Spain and Rome could turn out to be a profitable alternative to the Red Sea route (which wound up being a ferry from Berenice to Mecca, and a caravan from Mecca to Adana (Aden) before a quick sale across the Arabian Sea with the monsoons to India's Malabar Coast and beyond.. Even if it might mean dhows pilingup in Britsh or Spanish ports up for sale.
Of course if our navigator stopped at the Sengal River and rowed up it to it's head of navigation instead of trying to sail forther, he would encounter little disease an, the Niger River and the gold of Jenno-Jenno--which might invite conquest. With Djenne jenno as a draw, a caravan route straight across the Sahel to Abayssinia and the Somali Coast might be feasible--and might also give Rome the incentive it needed to conquer the Garamantes and build a viable road across the Sahara from Leptis Magnus to.o, as wel as conquer Nubia and possibly even Abyssinia.