For artillery shooting (especially howitzers, mortars, etc.) do not need LOS at all. Moreover, in most cases, artillery fires to those places that it is physically impossible to see from its positions. This provides the difficulty of its suppression by the enemy. Moreover, by shooting one point you can transfer the fire to another, not visible to the observer (the spotter). Similarly, artillery works in our games for the player and for AI. And this moment will always remain so, only the form can change.

As for the density of artillery. In Tunisia, it is physically smaller amount (part of the division artillery), while the area is larger, and the density of troops is less. At the same time, American artillery is practically inefficient, due to problems in troop management and communication..
In a Mius, at 3-5 times smaller territory, the artillery of the two armies is concentrated. Naturally, its action is much more significant. Actually, this was the means that allowed the offensive, the rest of the troops were finishing off strong points and survivors. It's strange to complain about her action in the game.