Oromo Warriors are definitely not the worst unique unit. They are actually the best Gunpowder based unique unit. As already mentioned, they are best promoted to Drill IV which is possible with Barracks and Theocracy, Vassalage or Great Military Instructor. They dominate all pre-gunpowder units and are a great complement to Cannon seige units. There is no need to wait for Rifling to upgrade them to Riflemen, although such an upgrade is one of the best.
That's not true though, knights ignore first strike and I think, can get pinch so they counter Oromo hard (bet the Italians feel silly now)