The eighteenth century being a time of standardization and mass production also leads to it being a time of soldiers all looking the same. There is a certain appeal to that clean and efficient look, but it also gets boring. All of these units are just higher stats and a different look - not really any unique mechanics, but they make the game look more interesting.
Only the British and Americans really get anything from this and many of these are region specific which make them pretty interesting to have (although harder to replenish).