Spain is a case where it's sort of better in the hands of the AI right now. As a human on high difficulty it can be tricky to stay above the happy curve, and as someone was saying earlier a bad pop hut early can put you into unhappy on the 4th citizen and it's pretty much gg with the reduced settler training speed. It reminds me a bit of some of the pantheons that are stronger for high-difficulty AI, but being a whole civ it's a little disappointing.
I guess maybe you could try going Worker First instead of Monument->Warrior->Settler to deal with it better, but that also requires a 1-tech lux and probably some good farms to really make it worth it.
Yep, if the happiness system remains as it is Spain will probably need an adjustment. I've tried a few starts as Spain and its consistently a problem. I've adjusted to an aggressive early lux strategy with them on Emperor. It helps somewhat, but I still have hit the happy limit in the very early parts of the game.