I've had this problem for a long time across multiple total war games ever since we moved from ME2.
Missile units especially artillery, and in warhammer the flying units, and big shooters like ushbati greatbows, 99% of the time they just target a select few units in the corner of an enemy battalion.
For example, enemy unit of darksharks is standing in a position somewhere in front. You fire a few rounds of cannon fire/ magic missiles at them, then all these shots simply land of the 2 right most column of the enemy, wasting a bunch of shots to kill like 3 or 4 of them.
Quite often if there are one or two stragglers away from the main formation, the missile units will shoot at those instead of the main formation!
Imagine this, you have your "trusty" missiles, be it a unit of terradon (firebomb), ushbati bows, whatever. You see a unit of 150 skaven runners coming towards you. Instead of firing into the middle of their formation and break them apart, your missiles all aim at that 1 slowpoke rat that is running some few meters away from the rest of the group. You wasted precious ammo and crucial timing and now the enemy is wrecking havoc. JUST WTF! Occasionally, missile units plain refuse to shoot at a blob of enemy just because the one unit that they are targeting has line of sight blocked, all while the main blob of enemy(same battalion of enemy) is just straight ahead!
I can't believe this issue has not been addressed across so many total war games, we never had this garbage back in Medieval where missiles properly land in the middle of enemy formations!