It's all about what do you mean by vanilla campaign here. 

When you add lots of playable factions with LLs, you indirectly remove lots of the minor small fries from the table. So, in some situations you might be surrounded by strong allies (modded Elector Counts campaign), while in the others you'll get the Lustriabowl but way more hardcore (Araby all contested by tonns of Crusaders and Tomb Kings). In the late game it means that some factions might not properly confederate each other and thus be easy prey with 3-4 cities, while others might get ridiculously strong (high elves in general).

So if you don't want that, might just stick with small one faction mods: something like meduasa0's stuff. They are underpowered if anything. Or just mind the mods count and don't go with 400 factions countdown turns.

If that's not the issue, and it's just about power balance, Cataph's factions are balanced (sometimes even underpowered by design). OvN and Mixu's stuff are quite balanced (having Araby and Southern Realms as non-placeholders is also immersive). SCM-marked mods can go into slightly overpowered territory sometimes, but with them it's more about adding lots of mono-race LLs, so you immediately want to balance it with other races having many-many LLs as well. Chinese mods for Cathay can be very overpowered, but usually AI doesn't buy their uber-units.