The early muskets in game, are rare , because it takes a month to train them, their very expensive compared to say crossbows which are basically free (as levies and trained up) and you can only train up to 20 at a time in a town, so unless you used several towns or castles to make an army of them, also in the field they aren't that much more effective than say elite crossbows, except the fact that they have very good range, but I noticed if you don't protect them well and they get attacked that they are very weak against any infantry and cavalry. Also I noticed they tend not to take out their swords when confronting the enemy face to face, and this makes them die even that much easier. All in all I think they are balanced well and not overpowered at all.