1. Ranged troops don't really cause any friendly fire in this game. They do in other total wars but in this one, you really only have to worry about friendly fire from artillery.
So go nuts with putting your gunners behind your lines.
2. If you have trouble with artillery positioning, try splitting your army apart. If you have a lot of artillery, it can work to have your armies in batteries like Napoleonic armies. Have them all cover each other so no matter who is fighting, there will always be artillery fire.
The AI doesn't really know how to handle multiple batteries so it'll usually only ever attack one with maybe one or two light Cav attacking another.
3. Handguns are 3x more powerful than crossbows. Armor piercing damage is true damage and works against everything. They melt enemies. Crossbows only advantage is that it shoots over people's heads.
This isn't that big of a deal as you can arrange your formations to have little gaps in between them for your guns to shoot through. Or you can position then sightly higher up. Again, they don't cause friendly fire so who cares. Additionally, they simplified formation shooting in this game so even if your handguns are in square formation, they still shoot at maximum effectiveness.
Also, if you use a little micro, a flanking barrage will completely rout an enemy.
If you don't believe how brutal handguns are, go into skirmish. Subterranean battle. Put 1 army of just handguns against 2 armies of any greenskins. Armor piercing is ridiculous in this game.
I suspect it might also have something to do with how crossbows also miss a lot when enemies are moving but guns don't.