Rocket Artillery Vehicles really should be doing less, overall, than Field Artillery pieces.
Especially considering the mobility of the former.
I think a rough benchmark of Field Artillery is a good thing to keep in mind when considering how "good" Rocket Artillery Vehicles finally even out to be.
Static shell artillery: Offmaps and field artillery range are two things in which one makes it useless and the other keeps it from being useful (other than static artillery being static and doctrinal). I see no reason how a giant spray of rockets could be less effective than one artillery piece firing a barrage. All artillery can only be effective in numbers, but this game favors rocket artillery because it can do better on its own than the former.