It's really, really simple.
PzWerfer and Calliope, they wipe squad behind green cover, in trenches, in wood buildings and in stone builings.
Katy and Stuka are very useful BUT they don't wipe squads in stone building (or they give you a chance to set up HMG and get out) plus they give you small chances for survivng volley on the open space.
It applies to Katy, Stuka, PzWerfer and Calliope barraging from the same distance.
Brits
So tell me one more time that PzWerfer and Calliope are not OP.