I think the main reason that the arty was OP in vRen is that it fired shells instead of GDI's MLRS missles. This is because the shells travelled significantly faster than the missles, making the shells harder to dodge. This may no be so bad on smaller maps, but on larger maps like field where long range arty-mlrs duels happen, the artys shells are so much harder to dodge, putting the MLRS at a disadvantage. Not to mention the MLRS fixed turret. You could, and still can in REN X, drive the arty backwards (With no speed penalty?) to minimise the area the enemy has to shoot at, as compared to driving forwards with the long body sticking out. Besides from experience, being in an arty against a med tank gives you a better chance than being in an mlrs against the light tank.
Still, in REN X, the MLRS has a fully rotating turret, and personally the missles seem to travel slightly faster (Correct me if I'm wrong), evening the odds.