90% of what OP has said in this thread can be applied to PWerfer or Walking Stuka to a lesser extent.
That may be true, but especially for the Panzerwerfer it is a different thing in my opinion (only in the case that you can call in the Calliope without any tech requirements - I really don't know if that's the case - ).
I think so because the Panzerwerfer is indeed really strong, especially in team games, but in 1v1 you won't see it unless your enemy has so much fuel that you would have lost the game either way. Ost T4 is mostly bullshit because it's just not worthwile. You spend so much fuel and MP for teching and building a Panther will still cost you 175 Fuel for every single tank, that's simply a lot.
The whole USF commander is the problem. The Calliope wouldn't be that OP if the rest of the commander sucked. But everything in this commander is pretty good. The LMG is amazing and makes the rifles the best inf on the field. How do you counter massive inf blobs? With MGs - well, at least it is supposed to work that way. So the Ost player is forced to have at least two MGs to counter the blob of doom.
But wait, you have to prepare for the rushed Stuart (or a Quad, which is also devastating)! So T2 and a Pak is needed, which is also a lot of MP, so you lack field presence. The USF player though, has about 4 Rifle squads (some with LMGs) and a captain/lieutenant until the Stuart arrives. The Ost player probably has not more than 3 grens (probably 2), 2 MGs and a Pioneer. The USF player at that time holds about 70% of the map. Then the stuart arrives and the Ost player can't do anything but to get a PaK as soon as possible because of the lack of light tanks. Meanwhile the rifle start planting mines like fucking flower pots everywhere. The USF LMG simply shreds through your troops and smoke negates the effect of your MGs/PaKs... I don't want to continue, I just want to explain the fact, why Ost suffers so much against the ultra mobile US army. Ost has NO OTHER option than PaKs to counter fast vehicles and NO OTHER options than MGs (maybe mortars but they suck a bit atm) to counter the rifle blobs.
And that is why the Calliope is so devastating. It kills the stuff Ost has to rely on with one single barrage with little to no counter play.
Meanwhile, I just go and hunt down every Calliope unchallenged with a Panther
That simply doesn't work. As Ostheer you are crushed if you don't get some Anti Inf tank as fast as possible. There are about 5-7 USF inf squads with M1919 and bazookas and smoke so your MGs are useless. Then you get a Panzer 4 and try to get that fucking Calliope, but then you remind yourself that it's not a Katjusha that dies after one shot, but a damn M4 Sherman (you probably hit a mine on your way anyway
), so you have to be satisfied with hunting the inf blobs (where the P4 is pretty good at). You even might be able to get that stupid little Stuart. You position your MGs very well and you can manage to get a second PaK, but then woooooosh - another Calliope barrage and all your weapon teams are either dead or with 2 men left.
TL;DR: Ost has no other way to survive than relying on defensive units such as PaKs and MGs, which are wiped by a single Calliope barrage. USF doesn't rely on defensive and immobile units, so the Panzerwerfer is maybe a bit too strong, but no gamebreaker like the Calliope.