It is very hard to aim the naval artillery because of the big height difference between the water level and the positions on the cliff. On top of that, if you just overshoot the cliffs a little, the rounds often land in the fields behind the trenches, outside of the artillery view radius, which makes it hard to adjust fire. Last but not least, once the americans have gained a foothold in the trenches, it becomes very easy to teamkill with short rounds, since the situation is chaotic and the lines are only metres apart.