Stuka is easy to dodge. Calliope is not.
Yup, calliope costs more what what if you need arty to counter vet 3 double LMG rifles? You need to tech and teching to T4 in 1v1 is not cheap. Meanwhile USF player can get non tech Calliope and spend rest fuel of Jackson/Shermans.
Well vet3 lmg blobs is a problem for another thread.
Thing is the pwherfer was designed as a T3 unit, it was only moved to T4 because the T4 was barren after losing the elephant. And TBH the idea of "T4" unit being stronger is something that never really worked out. Since some factions max out at T3 level tech, T4 can't actually be really stronger because at the end of the day the T3 level factions still need to be able to handle it. So in actuality T4 ends up just being kind of different rather than actually being more powerful.