In my opinion there should be less rocket systems on both sides. 4 Pieces of any kind max per deck or something like that. So that they are prized possession that will give you an edge, however you have to be careful how to use them. Granted Allies have an advantage of their systems being strapped to vehicles only, while with axis it depends on the deck, but still.
I think that having them few and not corrected by reccon should be enough of a nerf to them. Regarding power, we should be careful so that we don't get them so weak that they will be laughable... like some MLRSs in wargame.
Ju 88 is the best unit vs arty.
Provided that you manage to reach the target.