When Total War 3K launched, militia spam was widespread and notorious. They were the most powerful units in the game. This was because they had far lower upkeep than every other unit but were fairly solid at holding the line, making it so your generals could have ample time to wreck the enemy.
CA patched the game to make militia weaker by significantly reducing their morale. However, they fundamentally misunderstood why militia were so strong in the first place. It wasn't primarily because of their stats, but because of their cost efficiency. Professional units are just way too expensive seeing as your generals do most of the killing power. Instead of paying for five professional units you might as well have eight militia ones. In Three Kingdoms, the numbers of your army are often more important than quality, since the main purpose of your army is to pin the enemy down while your general charges in and wrecks them with their abilities. Sometimes, you, and most crucially the AI, also cannot afford armies that aren't mostly comprised of militia because of how tight the economy is. Especially in the early-mid game.
This has resulted in fights against AI armies mostly made up of militia who, after the patch, route very, very quickly before battle even begins because of how low their morale is. If you play with night battles, fire arrows, and generals with scare, you can literally win battles with almost no kills. I think the game was better when militia were sturdier. The problem of militia spam hasn't gone away because their strength is in their cheapness. Instead, the game is still mostly comprised of militia armies but now generals are even more overpowered because they can very easily route the enemy army, making battles somewhat of a joke.
The frequency of militia spam was increased when CA decided to drastically cut recruitment cost (a good thing) but increase upkeep costs (a bad thing). Some faction exclusive cavalry units cost over 400 gold, which is frankly ridiculous. Instead of fielding unique armies, the AI, and most players because of monetary purposes, will simply use 2 militia cav or 4 militia infantry instead. This upkeep adjustment patch vastly increased militia spam in Three Kingdoms and made the battles even more broken as a result. The game now is almost entirely dependent on the strength of your generals until the late game.
This has also resulted in some professional units, most noticeably spear guards, archers, and crossbows, which are far sturdier in morale, stats, and are reasonably priced, being completely overpowered and used in nearly every army composition.
The way to fix this is to increase the morale of militia units so they don't break almost instantly after engagement. Reduce the cost of professional and faction exclusive units so they do not completely drain your economy. They should not cost almost twice the amount of militia, as is the case with several units and especially cavalry (thinking of heavy xiliang cav and Chen peacekeepers especially). I'm aware that elite units like Jade Dragons and Azure Dragons should be expensive, but we have a weird state now where there are militia which are affordable and everything else which is far too overpriced. Then there are exceptions like spear guards, archers, and crossbows, which are broken as a result of their reasonable upkeep.
And before you say something lazy like "get good", this applies to the AI as well. Because they find it hard to manage their economy, they field mostly militia units, which is a very, very large reason why the battles are such a joke.