+ a government needs to be discovered with a tech, and if this tech is available only to a limited number of civs, then you can restrict the number of civs who can build the corresponding building.By government
If you wanted a certain side to be able to bulid something in a war but you aren't too picky with the specific civs, you could assign the required government to be Chaos for an altar of the Great Evil Spork. Thus only evil civs with the govt of Choas could bulid that. You would have to assign "era:none" techs to allow for govts to be assigned to the civs, but once the govts are in place, the wonders and improvements with strict requirements of govts would then only be able to be produced by specific civs with the correct govt, and those buldings would actually cease to function under a different govt. This can prevent a conquering player (that has a different govt) to take advantage of the unique bulding.
This can be used for a little competition between the various evil civs: which one will have the bonus units or any other benefit?