The Numidian Cavalry is an excellent UU. The bonus it receives against Melee units more then makes up for the loss in strength. It makes an excellent unit to clear the path of enemies, allowing your main city attacking units to have clear path to the city you wish to attack. The Gallic Warrior does seem a little useless however. I've not played as the Celts yet, but it does seem very underpowered for a UU. Even the Jaguar seems better on paper because at least that requires no resources and is cheap to build.