Historically speaking, there is precedent for the National Limit on the crossbow.
Pope Innocent II banned the use of crossbows against christians at one of his religious councils. There was a solid reason for this. A crossbow required very little training or investment. It could however allow a single discontent peasant to kill a knight who had trained for most of his life. This would not only weaken the church against its religious enemies, but would remove a source of funding for it as well.
As for the widespread use of musketmen, I would like to see some kind of mechanic/function that caused their power to fluctuate randomly from turn to turn or battle to battle, to reflect the experimental nature of gunpowder. Sure, they *could* be the most powerful unit on a given turn. Or they could be wiped out by a group of axemen.
Again, there is some historical basis for this. Most early gunpowder units were highly inaccurate and vulnerable. Their chief advantage lay in how quickly they could be trained compared to Longbowmen. Really, the first point-and-click interface. However, the lack of training meant that they were pretty much useless in close battles. Also, this is prior to the invention of the socketed bayonet, so they essentially would be wielding large clubs. Because of this, most early formations relied on the heavy presence of pikemen to strengthen the formation. Units like the Spanish Tercio used gunpowder to shock the enemy formation and pikes to crush through it. It also made the gunpowder unit far less vulnerable to cavalry charges.
So, gunpowder units were poorly trained [some notable exceptions including Gustavus Adolphus's forces]. They used unreliable technology, which could be rendered useless by prevading damp or light rain. And they required significant strengthening with pikes to become real forces on the battlefield.
Suggestions:
-Tercio National Unit. Available at Gunpowder, Warfare and Mithril Weapons [or where-ever phalanx is at]. Sacrifice a Phalanx and Gunman to create a Tercio. It receives a bonus against Cavalry and in open-ground combat [anything but forests and jungles], with a penalty on city attack. Ups the base power of Gunmen and removes then new penalty mechanic.
-Lower strength of Gunmen to 9, make them slightly cheaper than Champions. A better choice if you don't have mithril, but a slightly worse choice if you do.
-New mechanic. "New-fangled" [until someone comes up with a real name]. Functioning like the old/new crazed mechanic, has a chance every turn to grant a promotion to the unit.
+Bad Weather: Lowers strength by -5. Flavor: reflects rain and damp making their weapons essentially expensive clubs. Low chance of occurring.
+They expect us to what?: Lowers strength by -2. Flavor: reflects the low levels of training and morale found in the early gunpowder armies. Moderate chance of occurring.
+Look at 'em run!: Raises strength by +1. Flavor: early firearms could often break opponents who were unused to the mid-range devastation of a proper volley. Moderate chance of occurring.
+The whites of their eyes: Raises strength by +3. Flavor: Under good strategists, gunpowder could carve a swathe of destruction across conventional forces. Low chance of occurring.
This makes heavy investment in Gunmen a gamble, as it should be for a fantasy world experiencing the first scientific revolution. While your uber-promoted man may be a terror on the battlefield when its sunny out, he's next to worthless when Armageddon causes a few sprinkles from all the evaporating lakes. The penalties are weighted down more than up so that you are encouraged to move your gunmen with support from other units, just in case your entire stack decides today *isn't* a good day to die.
