OK.
you also need an effect that effects Victory directly. Makes it harder to win if the enemy has this weapon. VC is the deciding factor for wining or losing the game, not losing a Major and going to a Minor which means you can place 3 units after paying 6 IPC. The effects of this weapon destroy the resolve to carry on the war. It effects the national morale in that they see the enemy has a great advantage of destroying entire cities and killing 100,000 people in seconds. It is a Sword of Damocles that cannot be overcome. The resolve of the nation sinks as does the entire war effort.
Just have it effect Victory conditions, not freaking rolling for destroying countless armies in a giant area when the effects would be of a small area. You drop a bomb on Germany and it has 12 infantry, 4 tanks, 3 artillery, and countless other units. Even if targeted against an armor division, you might take it out but to assume all these other units are stuck in the same 5 x 5 mile area is silly.