Not sure what to do with the Nords but the solution for the Rhodoks is absurdly simple: arm their archers better.
The one thing really going for the Rhodokx was that getting in close with their archer class wasn't a guaranteed win. The Rhodok Sharpshooter could handle himself if it came to blows because he had a military pick or another weapon of a kind that allowed him to do some damage, plus the huge board shield that all Rhodok archer types apparently still have.
You can't go pure archer specialist and arm the Rhodokx with daggers and short swords. Doesn't work. The unit class and the engine are not balanced to allow that to work. Rhodok crossbowmen need to be able to hold their own when (not if) cavalry gets through to them. They shoot too slow to prevent closing, and Rhodok infantry is too slow to hold off the first wave, if the first wave happens to be Ecruyers, Timariots or Morici's. The fact that Rhodok infantry utterly SUCKS doesn't help either.
Also, each nation needs at least one unit that's as powerful on attack as the Vaegir Militniy Druzinnik, which is ridiculous. Druzinniks are incredibly unbalanced with powerful, quick heavy hammer weapons and no fear of using them. That entire chain starting at the Grid' is just a little too effective, and it gives a hint for how to answer issues with other underpowered infantry types -- more hammers or one handed axes with a high speed rating would go a long way. In the Rhodok's case, the Military Pick and Spiked Staff are canon weapons that would go a long way to the positive.
The fact that's being overlooked here is that Piercing and Blunt damage are better at getting through armor than ordinary cut damage is. Swords are great against unarmored units, but only top of the line swords are all that effective against anything with a body AC of more than about 35. Piercing and blunt damage (and axes that do extra damage to shields) are better bets against the kind of enemies that really bug Rhodoks and Nords.
Also with Nords in particular you really want to stop screwing around with lightly armored base infantry. The armor might look period, but it doesn't stack up, especially for a race that ignores cavalry altogether and thus should if anything be BETTER armored. Having unstoppable T7 Nordish jugggernaut units is fine but the AI won't use them. Some kind of stalwart, hardy core class taking up space in the T4 area somewhere, like the Militniy Druzinnik for the Vaegirs, who I note has Studded Leather and thus outclasses by 10 armor points most of the T4 Nord units I saw.
Similar issues exist among the Khergits. It is entirely too easy to take a city from the Khergits with an army of Druzinniks and then hold it indefinitely from wave after wave of Khergit sieges. The reason is simple -- most of the really decent Khergit units are beyond the range where the AI will actually use them. It's nice to have 'em on the tree, but they don't really effect the balance properly if every good Khergit fighter is T5 or higher.
So if I had 3 suggestions, they would be as follows
1: Nerf the Druzinniks. I love 'em, but they're too much of a one-size-fits-all solution as they're tough, reasonably well armored, and boast that powerful hammer. They need an armor downgrade to justify everything else they have going for them.
2: Military pick on the Balistere and similar. If you want to go whole hog and put a weakish hammer on 'em fine, but don't think "Rhodok crossbowmen. OK, archers. OK, dagger." In their unique circumstance that does NOT work
3: More armor and/or bigger shields for the Nords. PLEASE. I don't care if it doesn't look like a historically accurate viking, because you know what? The vikings LOST, and they aren't here anymore to defend their side of things. No one is going to be mad at you for historically inaccurate vikings. Nord fans are going to be mad at you when the AI on their side boasts nothing bot a swarm of sheap sucky spears and armorless fodder. Survivable Nordish units need t start somewhere at tier 3, please and thank you. The idea that Nords are owned that badly by Vaegirs in castles where Vaegir mounted advantage is weakened is just lame.
That's all, hope there's something there you can use.