Okay, I ran through the early stages of several campaigns. Here's just a miscellaneous run through of things I encountered.
- "Unrest" is a pretty big problem for a lot of factions to deal with. I don't recall it being a major problem with vanilla RTW so I assume its something thats been modified in. Its not really fun to deal with with any group, but certain factions lack big bonuses to add happiness to settlements or cheap units to sit in cities. A lot of the "barbarian" factions have this problem. This is harder to tell without playing a full playthough, since I also noticed buildings for Barbarian factions were not showing up past the third tier (Dragon Islander's temples are an example). Factions should really have some means of getting at least +75 order in a settlement, even if its a pricy/slow means to do so. Wood Elves in particular look nigh unplayable in the long term. Factions with split starts also have major difficulties with disorder due to "distance to capital" issues.
- Shields are pretty inconsistent. Most units with shields have a shield value of '1', which is basically nothing. Dragon Islander's Long Shields have 5 which is the highest I've seen. Several units do not have shield graphics but have shield armor value, such as Djinni and Knight Errants. Many others have shields but have 0 shield value such as Eastern Spears and Hobgoblins. Shield values in general seem to be either low or nonexistant, which makes missile troops very powerful. I've had AI units dissolve while trying to mount a charge into a group of archers. Wood Elf glade riders are also very powerful as well.
- Speaking of the dragon long shields, they seem to have a very high ammo count for their very damaging projectiles. I would imagine since they're pretty heavily based on legionaries, they should only get two. In any case, they basically annihilate any infantry that isnt a berserker unit.
- There are a few units that are very, very powerful, but honestly they're such a blast I don't care. Guys like the the djinn, bright wizards, giants, and those Chaos Dwarf mechanical bulls should have high cost and upkeep though to counter the whole "we can destroy an entire army solo" power factor. They're hilariously fun to watch in action, in any case.
- A few more creative units don't seem to work well with the engine. Araby flying rugs cause problems when they die where enemy units keep trying to fight with them. Landmines in general seem sort of wonky and just don't really fit in, and Walking Woods... well, they're funny when they're running away, in any case.
- As others have noted, there is virtually no income from trade.
- Some of the AI factions are very passive. Skaven, Undead, Dark Elves, Orc, Beastmen, Lizardmen, and Kiev do not expand much at all. Chaos expands quite a bit, but they take a long time to take over the rebel settlements and come into conflict with other factions. I don't know what triggers AI expansiveness, but I think the lack of income from trade and high disorder caused from distance from capitals might have something to do with it.
- Undead probably should start with economic buildings. Their expense of their generals means that they start off with a very tiny income and them and wood elves are the only two factions where the AI attacked my initial provinces. Their start is very difficult. I'm not surprised the AI can't do much with them.
- Generals gain an excessive amount of traits.
- AI Tomb Kings run roughshod over AI Araby very quickly every time I play.
I'll add more to this post later as I continue playing. I know there's stuff I forgot to write down. I'm having a lot of fun with this, I hope you continue to work on it.
e: Also sorry if some of the stuff I'm writing are things you can't modify at all. I know there's a lot of things hardcoded like the CAI and other mechanics, but I don't know how much you're able to influence or mitigate, so I'm just writing whatever I encounter that seems off.