Cities do not revolt any more. The details of unhappiness are detailed in the article: slower growth. If your civ is very unhappy (no indication of how unhappy that is): "your cities stop growing altogether, you cannot build any Settlers, and your military units get a nasty combat penalty."
In any significant empire (Rome, Britain) trouble in the far reaches of the empire did eventually cause issues at home - There is the substantial cost in resources and manpower required to administrate and assimilate the troublesome territory; there is the social unrest caused by an increasingly diverse and disparate population; and expansion and growth inevitably widen the gap between the haves and the have-nots (since the wealthy/powerful are better able to reap the rewards of new opportunities).
There are plenty of justifications for the Civ V happiness model.
In any significant empire (Rome, Britain) trouble in the far reaches of the empire did eventually cause issues at home - There is the substantial cost in resources and manpower required to administrate and assimilate the troublesome territory; there is the social unrest caused by an increasingly diverse and disparate population; and expansion and growth inevitably widen the gap between the haves and the have-nots (since the wealthy/powerful are better able to reap the rewards of new opportunities).
There are plenty of justifications for the Civ V happiness model.
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