I believe the current American UP was a direct response to America's happiness problems at spawn, which lead the AI in particular to adopt Monarchy as a solution. I noticed when trying out Australia that happiness is still definitely an issue for late spawning civs, so the American UP still has a place I think. Another suggestion that was tossed around at the time was letting the Gold 'slider' provide happiness like the culture slider does, but this wouldn't do anything for the AI, which I believe always runs 80% research and 20% espionage. It also no longer has the synergy between a high gold slider and rushing infrastructure through the old Capitalism civic, since that effect is on Public Welfare now. I personally have no issues with the American UP encouraging the four civics that it does, and it has the effect of providing +8 happiness, which is huge for growing cities and offsetting Corporation unhappiness.
Some UP's I don't like though (all speaking in context of UHV):
- Netherlands. Very appropriate thematically, and a very strong UP, but the placement of Trading Company in the tech tree means you never really use it for the UHV. In fact, you can win the UHV without researching a single new tech. The UB has this same problem, in that they both come too late to put in serious work for the UHV.
- Byzantium. Also pretty appropriate thematically, but I tend not to use it for several reasons. First is that it costs a decent amount of gold to bribe, and you need 1 spy/unit bribed. I always think the UP is strongest pre-1000 AD for all the barb horse archers that spawn in the Balkans at that time, but to use it goes against two other things you need to be doing at that time: massing gold for UHV 1, and building real military units and not spies for the Seljuks. After 1000 AD, you have a bunch of gold to dump and the UP would then become useful, but barb spawns noticeably decrease at that time too, so it becomes useless again. The extra gold from diplo trades is okay, but nothing too great since you have very few trade partners around (Spain and Arabia both care too much about religion).
- Khmer. Takes far too long for workers to build a farm on a rainforest tile, and your prize is a 2F/0H/0C tile. Haven't tried it yet with the +1H change to vassalage, which might make it a bit more useful (2F/1H/0C) (assuming use of Caste System rather than Manoralism), but its just better to whip a harbour out and then work 2F/2C coast tiles. Doesn't help that there's maybe 5 tiles total that the UP can be used to farm, most of them around Pagan.
- Korea. Helpful for the UHV, but pretty boring all-in-all. Just kind of something that's there.
Aside from these, I think most of the UP's are pretty great, and tend to be helpful both for the UHV and general play, and are thematically appropriate. The other two mediocre ones are China and Maya, since they both just offset civ modifiers in rather non-interactive ways.
and improvements adjacent to two core cities +1 
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