Civilization – only get 4 turns of Golden Age for Reformation when it says 6

The base number of turn for the Taj Mahal is 15 a turn Golden Age. The Chichen Itza world wonder will increase that number by 50% (= 22 turns).

Building the Taj Mahal as Darius resulted in a Golden Age of 33 turns (should only be 50% longer). Darius with Chichen Itza resulted in a 49 turn Golden Age.

The base value seems reasonably balanced to me, maybe a bit too powerful. With Chichen Itza it is very powerful, but Taj Mahal with Darius is just outright overpowered, or even broken.

I would say that the only reason not to build on a hill is because there is a better spot. That might seem like dodging the question, but I guess the point is that while hills are good, they are not so good that there aren't plenty of situations where something else would be better. Maybe building off the hill gives you access to a resource you would otherwise miss. Maybe building on the hill keeps you away from the coast when you want to be able to build naval units. Or maybe you play a very Great Person heavy strategy and want to be by a river to build a Garden.

I almost always build my capital in the location that my settler starts, regardless of whether it is on a hill or not. Generally the starting location is dead center with a bunch of resources, and even that one turn of not building your capital does have a multiplied effect over time. So I would say that you should definitely take hills into account when planning city locations, as they are quite nice, but don't get so focused on them that you miss out on other locations that might be better.