I am currently playtesting and balancing the WotR campaign and it's coming along nicely. I have balanced AI stacks within a few turns. Below are some important things to look for:

Unitsize setting
I believe the AI trains units much more when the unitsize setting is set on huge. The reason is probably because the AI has been coded to have X number of men in a provence on default unit size. On huge unitsize the AI will then train 2*X number of men.
The AI will thus have twice as many men, but to also get twice as many units you will need to set column 21 to No. All units are now unscalable. Not only can you have twice as many men in a stack (still 16 units max though), you can also have alot of huge battles I think this is a big improvement compared with the many 8 unit battles in normal M:TW.

Unitcost
All my units can be build from a single building. Even when the build probability in column 15 was the same for all units I noticed that the AI built the more expensive units a lot more. Apparently the AI has been coded to build expensive units more than others.
This may seem all well since more expensive units are usually better but this is horrible if you want balanced armies In my mod Longbows originally costed less then mounted MAA. The result was that the AI built 6 cavalry and only 2 missile units, while it should be the opposite. I then ran a test where a Longbow unit was twice the cost and then the armies were mostly made out of Longbows.
My conclusion is that unitcost is a very important factor in balancing the AI.
However the bad thing of this is that you want the rare units to cost more, this will mean indeed less of those in player armies but more of them in AI armies. If you make rare units cost less you will see them less in AI armies but more in player armies since he will of course train the best for cheap.
I think the best solution will be making the units cost all the same and tweak with column 15 (unit choice).

Unitchoice
In column 15 you will see numbers for the different AI behaviours. AI behaviours change during the game. If you set a faction to be Catholic_defensive then it may well be Muslim_peacefull a few turns later So you need to balance all of them. I keep it simple and give all the behaviours the same number.
It seems that the higher the number the more the AI will build that unit. However the probability is much more influenced by the unitcost so the next test will be when I give all units the same cost and see what the effect of column 15 is.

Cheers, Duke John