Actually most civs UU's do. the only exceptions I can think of are the mounted archer units.
Anything with a straight Strength upgrade doesn't - both Greek UUs, for example.
That is actually a problem. One UU gives bonus right from the start trough out the game and the other UU (as you said it) in many cases forces you to wage war in order to take advantage of that current civs UU, otherwise it will be completely useless.
Those UUs tend to totally dominate their own era, to the point where the thing you carry forward is a bunch of extra cities and a bunch of extra experience. By doing it differently on different Civs, you get very different gameplay experiences across Civs. This is a *good* thing. Put it this way: If all UUs carried forward or if none did, which would you rather have: Jaguars, or Hoplites? Hoplites by a mile, right? Warriors aren't very useful already, and Hoplites are a stronger upgrade from Spearman than Jaguars are from Warriors, right? They'd have to nerf Hoplites badly to balance the two if they worked the same way.
I don't know if it was intentional or not, but I like it.


. I introduced an idea about a system for a civ game, not particulary for civ5 or for G&K, but maybe for future civ, like civ6. Then it feels like everyone is just nitpicking about some other things, it feels like nobodys truly understanding what I am saying here. Im not blaming you guys for it, since the problem is very likely in my way of expressing myself in english. I must admit that I do not have the time or the energy to engage further into a debate wich does not seem to go anywhere but also doesn’t even take place in my native language.
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(I.e. visiting family etc.) I hope we aren't being too hard headed but to put it bluntly I like the way the system is now where you have to balance your eras and buildings to a proper strategy. Also since you didn't notice this, I think the system does a good enough job of immersion as is.
. But thanks for the tip anyway.