I don't think they are to be honest. They're great for warmongers, no doubt, but I wouldn't say OP. I'd probably put Korea as the most powerful Civ in the game, that I've played with at least. To expand on that: the Zulu's major bonuses mostly all come at once, and they are ridiculous then, but it's not until the medieval era, and once the Impi become extinct or other civs get armies/corps, the Zulu aren't so special. So you have a short window to dominate other civs. Their other non-Medieval era bonuses are useful for warmongers, but they're not game changers. Their Medieval bonuses will likely let you run over your nearest neighbor or two if you don't have a terrible beginning, but you can't conquer everyone that way. An OP Civ has bonuses that are overly powerful all game and not for such a short window. Just my thoughts anyway.