Honestly, I don't think it's so much an issue of certain units being overpowered, but more an issue of the dominance of armor. There are very few units in the game that can stand up against these heavily armored behemoths, since the vast majority of units do cutting damage, and not always that much of it.
Though, in many cases, the answer is to use heavy ranged units, such as muskets, arbs and vilianese archers, but against cavalry this is a lot less viable.
Eh, I just think a few of the classic armor-busting weapons need a bit of an improvement. Two handed axes could use a speed improvement, and some of the polearms could use their damage increased a bit. Actually, more importantly, I think the light infantry units could use a boost to athletics. That'd be more effective.
All that said, there's still an answer to fighting these bastards. It's called tactics and choosing your battles. Never fight them in an open field if it can be avoided, this will get you killed. Pick rocky terrain, possibly with rolling hills. If you're lucky, the best location would be some sort of valley with large hills on all sides (in Calradia, an example of this battlefield is that large solitary mountain to the right of Praven. In fact, the whole system is about identical since the Swadians are a heavy-cav nation too.) as it gives you a perfect kill-zone. Next, you need two types of infantry (one with pikes or possible halberds, and the other with large smashing things. Such as conquistador pikemen and boar clan elite warriors) and some heavy-hitting ranged units, like those mentioned above. The best seem to be Vilianese longbows/nobles, rhodoks with their crossbows, BOUNDLESS SAMURAI! Although the samurai change things a bit, as they can count as your second type of infantry.
Now, first off you need to separate your infantry into two groups. One for pikes who stop the cavalry, and one for mopping them up as they flail around helplessly on the ground. Now, you line up on the top of the hill closest to your spawn point, you do not really have much time to do this. As you give the hold-position order, order your killing infantry to advance ten paces and spread out, and the pikes twenty and to tighten up. You might wish to play around with the hold-fire/fire-at-will commands if you're on a very large hill, but odds are these guys are already at you. The MOMENT the cavalry hits your pike-wall, order your killing infantry to charge.
If you had any cavalry, during this they should be with you moving towards the enemy rear, in an attempt to wipe out any infantry before charging in to hit their cav from behind. Well, unless they're cavalry archers, in which case you could've been harassing them the whole time. Also, if you had boundless samurai instead of boars, they could be placed in with the spear wall, or used as an entire front line if the hill is large enough. The front line can also be replaced with heavy infantry javaline-flingers, like those of the Antarians. They can take down horses pretty fast if they're on the top of a large hill, and have archer support.