Currently the Artillery Computer can only calculate high angle solutions (elevation higher then 45°). It would be nice if you could also choose low angle solutions instead (0 to ~45°). The manual visual targeting for artillery (that shows you the solution in the optic and you adjust it via page up/down) seems to rely on this and it only gives you high angles. Therefore you have many situations where you can't directly fire at something you see, even though you could point the barrel directly at it. So you have to trial&error this, which is far from accurate.
Some Artillery guns also have more limited elevation (lower then 70°), therefore they have a very large radius around the gun where they can't fire at all and they effectively have less then 25° for elevation variation, thus rendering the artillery computer useless for such guns.