The problem isn't that Cavalry are too strong, the problem is that they are able to do things which historically would get then whipped out.
Let's say you have 2 groups of Combined Arms (1 inf, 1 art, and 1 cav) facing off at each other.
As it is, you take your Cavalry and charge an infantry unit, which forces it into square. You then smash the unit in square with art and infantry and it routes.
In actual fact, if you charged a cavalry unit at an infantry unit or an artillery unit, the enemy would immediately counter charge you with their cavalry. A cavalry battle should ensue and your cavalry unit would be forced to win that battle first before continuing to make an attack on the enemy. Even if you won the cavalry engagement, your cavalry unit would almost certainly be disordered. Given that it's commander continued forward to attack an infantry or artillery unit in good order they should easily be able to repulse a disordered cavalry attack without forming square and very possibly routing if not eliminating it.
That is why Napoleonic battles were first and foremost infantry/artillery fights. Cavalry wouldn't be charged against fresh infantry or artillery because they would almost certainly be counter-charged. The infantry line would have to first be weakened for cavalry to be employed.
So if you want to attack an infantry unit with a cavalry unit and win, that infantry unit should first be disordered. Once the infantry has been disordered, you could launch your cavalry at them, but first have to win against a counter charge from the enemy cavalry if they are on the field. Given you won that fight, you should then be able to continue your attack into the disordered infantry unit, which would force it into square.
Employing cavalry first on a fresh field of battle in the Napoleonic Wars was a good way to get them destroyed. In this game, again if you attack first with your cavalry against units which are not disordered, historically they would be immediately meet with a counter-charge. If they continued the attack they would be annihilated by the enemy artillery and infantry. In battle, it is the infantry and artillery which should be opening each engagement. If you are successful in disordering your enemy, that is when you send in the cavalry. But in truth, you are much more likely to have cavalry vs cavalry fights (while the infantry and artillery are battering each other) to drive the enemy cavalry from the field. Once the enemy cavalry has been driven from the field then you can use them (especially heavy cavalry) to launch attacks on the infantry which because they are not counter-charged should drive them into square or on rare events, overrun the infantry/artillery entirely.
So actually, the whole battle mechanic of using cavalry as your first attack is completely ahistoric and if done so, should be heavily penalized as a move only an idiot would use on the battlefield.
Given that you don't get a reaction move in battles (in this case a counter charge) heavy penalties should be applied to a cavalry unit plunging itself into an opposing line with a combined arms bonus... strong enough that anything short of heavy cavalry should be eliminated. Once you impose such a penalty, then it only makes sense to require 2 infantry units + 1 cavalry + 1 artillery to get the combined arms bonus and your main attacking force should first be infantry and artillery with cavalry the last to attack to take advantage of disordered units. Even then, if faced with opposing cavalry, penalties should be applied against infantry and artillery unless the attack is against opposing cavalry units.
As a Napoleonic general, you should be first attempting to destroy the enemy's artillery and then second to drive his cavalry from the field; all the time doing everything possible to weaken the enemy's infantry such that once you have the advantage both in artillery and cavalry the enemy is unable to resist a combined arms attack, forcing your foe to rout and reap a deadly harvest in the pursuit.