Quite often when you are facing armour, its a good idea to place your anti tank weapons into concealed positions on the reverse slope of a hill.
This gives you the advantage of being able to engage at close range without giving them the advantage of being able to engage you at a distance that you can't effectively penetrate their armour.
It also means they cant spot you until they have crested the hill.
That's why its important to send infantry or recon in first to find out.
If you had a battery of 88's at your disposal then obviously it would be more advantageous to be on the top where you can engage at long range, if you had time to dig in and conceal them, that is.
They also have their soft under-bellies exposed as they crest the hill, and most are unable to depress their barrels enough to fire down from the other side without fully committing to the attack.
I'm not sure if this is modelled in the game though.
A lot of conventional artillery will also have problems bombarding targets on the reverse side of a slope, due to the flat trajectory, mortars obviously have an easier time of it.
The artillery observes will also need to be able to see the target,to adjust the fire, and will be forced to get close, making them vulnerable.
The recent updates have addressed some of the LOS issues associated with direct fire along and among slopes.
Discussion regarding the fix took into account the general slope between firing unit and target (e.g. if both are on the same side of a slope the relative firing plane between the target and firing unit is 0-degrees) and the only thing that would hinder direct fire between the two targets along the same slope would be a line of site obscuring geographic object (a secondary rise, buildings, or trees) between them. Same would apply to firing across a valley from one slope or hill top to the next.
Not being privy to the calculations contained in the fire resolution algorithms, I don't know how well the issues were addressed.
I believe the one thing which wasn't addressed directly was whether a direct fire weapon was firing uphill or downhill at a target; a circumstance which would affect the line of fire somewhat based on whether the round was flying down a slope with gravity or rising up a slope against it. The average slope and degradation or improvement in range was considered minimal because physical barriers tended to intervene in the line of fire sooner than the effects of gravity measurably affected accuracy.
There's a "minimum range" component in the Estab for direct fire weapons. The FLAK 37 has a direct fire minimum range of 20-meters for anti-armor and 50-meters for anti-personnel, which reflects how far the unit gun can depress its barrel until a target unit is close enough to get under the direct line of fire. Degradation of fire over distance (e.g. how far a direct fired projectile falls because of gravity as it traverses the distance between firing unit and target) is addressed via accuracy tables which reflect lower accuracy for each round at distance based on historically reported hit ratios per round fired from World War II after action reports and scientific studies.