[Mobhack]

Does that scenario have ammo trucks for someone? - ammo resupply units count as artillery points. Ammo bunkers can be expensive.

Does it have mortars? - mortars (and any other on map indirect capable units) are artillery and so count towards artillery points, its not just off-map arty that counts). Anything appearing in the bombardment screen basically tot up the arty points, excluding aircraft.

If you want to make them not cost arty points - figure out the arty units, and reduce their cost in the editor until the imbalance bonus goes away. Some scenario designers put in ammo units in order to go "boom" and ammo bunkers can be expensive, so if they are in "for effect", they may need their points reduced in the editor?. Some designers stack them on bridges for the demolition effect, for example, usually tied to a remote detonated booby trap.

Artillery overload is based on the overspend given the mission type. in an assault, there is no limit to the attacker (AFAIR). However, changing to an assault may make things dug-in which may be undesirable. Another thing to look at is the cost of units - if the scenario was designed long ago then units may cost less than if they had been recently run through the OOB cost calculator. So check an example squad and see what the cost is in the recent oob. Simply making the attacker's units cost more may whittle away at the overload bonus (e.g. make his HQ worth lots perhaps?).