This has been an extremely useful thread. I made an account here just to say thanks. Certainly explains why there is no list of corresponding UU and MCS.
Yeah I think it's awesome that they will still gift UU with resource requirements you can't meet. Might be worth scrapping a vanilla knight with 2 promos for a mounted UU. I had a game with someone providing longbowmen, which is always awesome for the double range promo.
You fellas mentioned towers. I may have been gifted one at some point. I certainly remember gifting them to city states myself as Assyria. I was using them as garrison placeholders until I was ready to upgrade. Good to build tons of them too because they still help in city capture actions all the way through the game, even if they die after being in range for the one turn. If you plan your movements correctly, they'll still help. If you have a perfect army and no significant terrain obstacles, their weakness in the field is irrelevant, as you can take the city on the same turn you deployed the tower. However they become priority targets for enemy air power. But you know, that can be a good thing too. Maybe that's something you can afford to lose at that point, instead of one of your upgraded star units from the early game.