Legit question: what is different between Katyusha and Stuka that they are on opposite sides?
You mean why the Stuka is on the "Has Faults" side? It is in my opinion a very expensive unit that can leave you wide open in a 1v1 scenario. The accurate line barrage is also a blessing and a curse: Good players, that quickly dodge to the sides instead of reatreating their units, can mitigate most of the devestating effect the Stuka can have. That makes it a powerfull, but in some cases a risky weapon if you ask me.