I think they're one of the worst unique units in the game. It's basically a glorified slinger or a machine gun the later which I never use.
It costs the same as a crossbow, yet dies to melee all the same, and for a bit more you could build knights which would be much better up close anyways. The other thing is nothing upgrades into them so you have to manually build them and it's not that impressive. And not too far beyond machinery, you're getting muskets and bombards.
If you don't have iron though, you can use it as area of denial near your cities, particularly hilly terrain. And if the enemy masses pikemen. Yea that happens.