first, i really don't know why you say they are OP after "Just faced one for the
first time and omg omg omg.".
unit stats:
hummel: max range 200 (set up) / 115 (mobile), min range 35, cooldown 60s (vet 0) / 43s (vet3)
calliope: max range 150, min range 36, cooldown 80s (vet 0) / 65s (vet3)
hummel is a real arty with high damage and range and quite accurate, while calliopes are very good for area bombardments, but have terrible accuracy unless you get them close to the target. when attacked by them, keep moving
it hardly hits the same unit more than once unless it's close to the target.
calliope is the only mobile arty for US and they can have 2 of them. WM have nebels and stukas, PE have hummels. when used effiently hummels can also be game changers. and when you keep moving and don't blob your units, calliopes are not a real threat
conclusion:
yes they are little trolls, but far from OP.
ps: i didn't watch your replay, but from your opening post, i can't imagine that he just did nothing and you had the whole map. he needs ressources and pop cap to have 3 calliopes (30 pop) + X riflemen...and all the ammo for the stickies... but guess i'm going to watch the replay now
just wanna see it
edit:
ok, i watched the replay and... :X
1. until the first calliope was called the game was quite even, with both sides holding about 50% of the map and you had more VP.
but you made a few lethal mistakes:
1. you used all your units in one group blobbed together -> easy target for calliopes, that's why casualties were so high
2. i saw those hummels parked right next to each other -> easy target for calliopes
3. i saw masses of unsupported tanks (panthers and panzer IV IS) rushing at his rifles (with sticky bombs+picked up schrecks) and AT gun -> bad idea and waste of ressources. even a lonely bergetiger trying to recover a wreck with rifles standing right next to it...
4. he used his calliopes quite efficently to shoot down every unit you used for capping
