I don't think aoe weapon spam would be an issue if they made the artillery vehicles have very light armour, and take time to deploy/un-deploy. That way if an enemy tries to artillery spam you with lots of those vehicles you know that a good portion of their forces are tied in manning the artillery and defending them (as an attack by just 1 or 2 gunships could destroy them all if left unguarded). I think it should be possible to attack/defend with a great variety of strategies and artillery fire is part of that, which brings it's own unique forms of attack/defence and counters to it.