This post is actually a follow up to my last post 'Map design, my No.1 concern with the game', here is the link if anybody is willing to read that one. https://forums.totalwar.com/discussion/comment/2428884#Comment_2428884
Just finished a double legendary campaign as Dong Zhuo in 77 turns. After occupying a few of SunJian's land and defeating one out of his 4 full stacks, he agreed to abdicate. I must say this a great mechanic saving me a lot of steamrolling time. A great mechanic like so many other good designs and huge potentials in the game, but they are unfortunately shadowed by this lack of player choices problem at the moment. I will discuss this problem with army composition as an example. This post could also serve as a guide in that matter.
Players after sinking a few hours in the game will probably find that although there are 5 hero classes, you only have several solid hero combos. This is partly because of the new retinue system and partly because of the actually quite small skill trees, but more importantly, this is based on the realization that all troops in this game can be classified into 2 categories: DPS - ranged and cavalry, and tank - melee infantry. Now let's discuss the great impact of this realization on army composition in details.
1. ranged unit. This is a simple choice. Strategists, that's it, you have to have one in each of your armies if you want your life easier. They have all the bonus skills for ranged and they are the only class that can recruit the time travelling Armstrong gun.
2. cavalry. This is a little more complicated. You have vanguards for lancers and commanders for sabers. Lancers are great for hammer and anvil but have zero ranged block chance except for certain special and late game units. Sabers have slightly worse charge bonus but exceptional in blocking ranged attacks (85 block chance with skill), in addition, they are a little better in defeating enemy cavalry. Considering your main threat on the battlefield will be the enemy ranged units and cav, I would go with commander, but honestly, they are both solid options.
3. melee units. The main thing here is that you need to understand that especially in early to mid game, they are recruited for their defensive instead of offensive abilities. You need them to tank enemy dpsers, thus shield for ranged units and charge reflection for cavalry. Therefore, although both champions and sentinels excel at melee fighters, champions are the clear winners.
Consequently, you have the following solid combos : champion/sentinel + vanguard/commander + strategist. If you want to further maximize, you will want champion + vanguard/commander + strategist, especially if you need the commanding skills of champions and vanguards. With this combo, you should defeat superior enemies with minimal casualties.
Honestly, since this is a single player game players should not have to min/max everything. It's just when certain options are obviously way better than the rest, I find it very difficult to justify not using them. This leads to a great problem though. Because at the moment every faction other than the yellow vest rebellions almost share the same unit roster, you would play everyone pretty similarly on the battlefield. I have discussed the map design problem in another post. Basically, because half of the map are either deserted or held by the Han Empire, a faction even more passive than the rebels in Medieval 2, and everyone is clustered in the northern part of the map, you will find that most factions also play very similarly on the campaign map - fighting the same enemies, conquering the same settlements (don't make me start at the immersion-breaking settlements' names, imagine instead of Rome you have Italia Town).
In conclusion, the game in its current state suffers from the lack of replayability because players are left with very limited choices. Some limits are hard as players have no say in it such as the map design problem whereas some are soft since players have obviously better options.
I really hope the future chapter packs will at least fix these problems. Less playable factions in later scenarios with more variable starting positions would be great. Possible barbarians faction packs will partly fix the army composition problem, although to really fix that CA needs to revamp the retinue system.