There's nothing wrong with the "infantry" or musketry. If you have enough troops and command their fire with purpose you'll do well enough. The real issue is that the enemy AI almost always charges straight at you which gives time for 3 volleys at best. The first two are not very effective because they'll be at longer ranges, the last one will be in effective range. So essentially you get one good volley aimed at the closest enemy troops unless you personally manage your lines' fire. If you have enough troops to have a left, right and center company, then you can get 3 effective volleys before melee. Plus whatever other divisions you have organized your party by.
Also battle size plays a role in achieving the kind of deadly musket volleys you expect. It's not going to be present in small skirmishes but if you siege a city and advance your line up to the enemy breastworks you'll see how quickly they get cut down.