My understanding of the role of the ARC is that it should be a tool to deal with heavily entrenched alien positions by forcing the aliens to come out and attack them. But right now I see it mostly use as a surprise/mass attack to take out a hive within seconds. Aliens don't have much time to react to that and even if they spot it early there's not that much they can do because ARCs have so much hit points.
I think the damage of the ARC should be reduced, together with their hit points. At the same time the ARC range should be increased and more emphasize put on its ability to shoot through walls (rather than waltzing 6 ARCs directly into the hive room).
This is the kind of thing that goes on behind close doors on normal game releases, where they are optimizing it right up until the end. You guys are just getting to play with a version of the game that normally only the developers see.