The Calliope isnt overpowering I think. Keep in mind this is still a tank, so it isnt going to just use the rockets on top to attack. It still has a barrel to shoot shells at tanks. Some other people are also saying about the Calliope not being innacurate. If you notice, the dice are fixed so that it gets less dice than the Katyusha so the probability of killing a unit with a rocket salvo from a Calliope would be the same as with the Katyusha.
The only unit I have real beef with is that Thompson. For 1 extra point, you get a crap load of stuff. 3 more dice on medium range, close assault, and gung ho. Thats farkin amazing how lopsided that unit is.