They should require line of sight to fire on structures.
It makes sense if they are mobile, especially in the quantity they can be created.
From an immersion standpoint it makes no sense to see them firing through walls anyway. That mechanics of firing through walls was just a holdover from the limited maps of NS1, where there was no other way to better simulate indirect fire support. It's not ideal that they fire through walls, and it's really not even necessary if they are mobile and backed up by marines.
From a gameplay standpoint it just has a lot of bad elements to it.
The arc rush trains you can't kill before they take out a whole room parked from outside of it, giving you no chance to utilize your defensive setups whatsoever.
The unintuitiveness of it, trying to figure out where the fire is coming from through the walls.