I think the UU is pretty decent, and it's certainly not the worst in the game, others come to mind like Slinger and the Naresuan elephant. The turtle ship is also a situational bad unit. Caravels are not really supposed to be used in naval combat (this is why they have the withdraw before melee ability). They are more for exploration and the 1 extra movement for Nau is a good bonus. Plus they also have the cargo ability, which gives you gold AND xp. If you start with armory and get some XP towards that 3rd promotion they can make really good ironclads later on. I wouldn't say that they have no advantage in fight.