... First off, I don't think fire storm was a thing(or means what you think it means). And secondly, if it was a thing, you are probably referring to things such as the Katyusha rocket launcher or various german rocket launching systems(most of which I can not name off from memory, especially one that I know was fitted to a half-track), which do not, in fact, fire projectiles(IE the rockets) that are on fire, except in the loosest sense of the word. They fire rockets which have a motor(the only thing that is actually on fire, cause otherwise it wouldn't be a rocket, but rather a lawn dart with a explosive body), with a aerodynamic body and a explosive warhead set in the front of the rocket. Probably not the best words used to describe it, but yeah, they are no incendiary arrow launchers.

They are concussion arrow launcher, Which brings back the question, is it better to destroy by explosion or by flame?

the BM-13 (also followed by the famous name "Katyusha"), was the most famous of the Soviet rocket systems, the sister of the Katyusha is the Andryusha, carrying less, but considerably bigger rockets. the Soviet rocket launchers proved themselves to be very deadly weapons, they were relatively easy to make and did an amazing job with dealing with the German army group B during the battle of Stalingrad


The Germans had their own rocket launchers, the half track that you mentioned is the Panzerwerfer 42, slightly less rockets than the katyusha, but slightly bigger ones too, only 300 were made, but they were effective enough where they did serve, the bulk of German rocket artillery was made by towed pieces, i haven't read about them, but i can only assume that they were good as well


basically, rocket artillery was devastatingly effective and used all nations in the war (maybe an exception: italy and japan), there were (few) cases where rocket artillery had even stopped offensives, one of such events includes the timeline around Stalingrad, Katyusha rocket launchers have saved the Soviets from a couple of German assaults, from german assaults in stalingrad to stopping an entire panzer army at one point in the summer of 1942

Rocket artillery is a strong type of assault unit, adding it to the game depends on people's opinions, i would like for it to be added, it was much more devastating than self propelled artillery, so the balance of such units would seem like a challenge, but you cant have historical accuracy and balance in one.