Any serious empire's going to have lots of military things going on on all fronts. That doesn't mean that they are unusually awesome at all of them. You could (with far more justification) argue for an American naval UU, for their Carriers (which are unquestionably head and shoulders above the offerings of any other power). Doesn't mean that there's room in the game for them, though. Even if there was another UU slot, I really don't think Roman naval power was exceptional enough to justify it. They didn't really innovate much in that arena, so much as they borrowed from their foes.