I guess that the designers thought that giving Sitting bull a UU that would benefit from the Totem pole and PRO would make it OP (There is no UU based on longbows, so they could have designed one).
Similar with the mongol leaders. One could have been CHAR to get faster promotions which would probably be historically as well founded as AGG.
The salon makes some sense historically because it was a culture/science thing. And both Louis and De Gaulle are not bad for culture games, so there is some synergy with traits. It's rather useless for non-culture-victories, of course.
Gilgamesh and Hammurabi give also odd combinations. Gilgamesh is the strongest protective early game civ (unrushable with CRE and PRO), but the vulture would probably have been OP with the AGG boni. Hammy has archers with a bonus, but the AGG trait that doesn't really go well with it.
Especially in warlords Wang Kon could in resourceless situations use the Hwacha with Pro-promoted longbows. Of course these stacks are still vulnerable to mounted units, but when siege could kill the Hwacha was devastating against ancient and classical melee. It is still an interesting UU, but very situational.
As for UUs, they are oddly distributed, many more ancient and classical than later ones:
Warrior Inca
Archer: Babylon, Mali
Axe NA, Sumer, Greece
Spear Maya, Zulu (Greek Phalanx in Warlords)
Sword Rome, Celts, Aztecs
Chariot Egypt, Persia
HA Carthage, Mongol
Catapult Korea
Elephant Khmer
Maceman Viking, Japan
Pikeman HRE
Xbow China
Knight Arabia, Byzantium (Spain in Warlords)
Musket: France, Ottoman, Ethiopia
Cuirassier Spain
Rifle England
Cavallery Russia
Caravel Portugal
Galleon Netherlands
Missing: Longbow, Treb, Cannon, Grenadier, all air, all other naval, all recon units, all industrial/modern except for Marine/Seal and Armor/Panzer
Interestingly, one of the least popular/weakest units, the Musketman has no less than three UU improvements (perhaps because it was so easy to improve...)