It's the first CIV I ever modded by the way, since it's the first one which turned out to be so broken, unpolished and just dumb at places. I wouldn't say it's *fundamentally* broken, but just the amount of hidden or obscure mechanics or wrong text made me mad first time I played it (like descriptions in unit upgrades, unit types, missing text strings in civilopedia and so on).
Mechanically and content-wise, it is an obvious AAA throwaway where all money went into 3d models of leaders (which are not even that great - CIV5 had guys on horses, inside rooms, native american sitting on a cliff with an eagle flying by, while CIV6 is just high pol models on same decorations), with things like actual interesting victory conditions, religion and politics left as barebones (fuck tourism, fuck archeology with pretty pictures, give me my UN even if broken, Morgan Industries economic victory and other cool stuff instead).
Modders even found amateurish mistakes in code to fix, so apparently these people can't even code the game right.
It is also important for difficulty. For example, Unit Rebalance - just switching Retainers and Conscription cards in time not only makes sense somewhat historically, but also changes game for the better. -1 gold for support in the Ancient Era where units cost, well, 1 gold, something seems wrong don't you think? Suddenly you can't spam Archers anymore, and your german neighbour has horses and you don't, so he rushes you with like 5-7 cavs, but you have spearmen and mod also gives spearmen +20 against horses, so I spam my brazilian spearmen and put them and archers into jungle while 2 crossbowmen that I actually DO have money for (because of trade with friendly England, without it I'd go to negative) slowly munch through those behind walls. Sure brave brazilian spearmen still die even with the bonus since cav is so good in this game, but for 1 brazilian dead 2 more rise from the cities hidden in the jungle and thus blitzkrieg fails and I manage to go into offensive later (in a sense, since after that offensive I had about 4 units left, one of them a bombard, because Germany with horses beelined to modern Cavalry which required at least a unit of crossbowmen and 2 pikemen to kill and I lost units left and right - Smoother Difficulty mod at work with it's bonuses to AI science).
And great thing is, other AIs weren't just doing nothing - they're all just barely behind my Brazil (with it's op amount of science I get from placing campuses all surrounded by jungle tiles) in Renaissance, while I'm and Germany at the beginning of Industrial. In unmodded game they would still fight some barbarians with sticks (and England which is now also in Renaissance was almost destroyed by barbs and Germany in the beginning of the game).
P. nice.
In unmodded game I wouldn't even bother with spearmen units, I'd just carpet my territory in archers/crossbowmen (probably crossbowmen) and forget about it. Of course later with increasing amount of markets you still get an option to carpet your territory in rows of units which makes any offensive by AI, unless he's ahead of you in tech, doomed, but at least now with a mod I have at least 1 more Era which is fun, as opposed to vanilla where everything happens during Ancient/Classical and then just snowballs from there.
One other mod I am concidering now is which would lock you out of Eras somewhat before you discover most of tech, since beelining just screws whole unit balance across the world and adds to whole retarded feel of CIV6. But for now I ended up just using Real Tech Tree which adds more requirements for techs and nerfs euricas a bit and modify 8 Ages of Pace to make things slower in general.