Pwerfer must be up close to properly wipe squads. come to close and you are one shot killed by pretty much anything other than small arms fire. Yes you can instantly move it back afterwards but you are still at risk while firing. They enemy can also send in any vehicle and hunt down the Pwerfer and kill it easily.
The Calliopie on the other hand can literally have LOS on the enemy, wipe them, and walk away unscathed. If the enemy try to rush youre calliopee with anything smaller than a p4, you can simply reverse yourself away taking a couple hits and still have plenty of time to have AT guns and infantry come snare the enemy vehicle before your calliopie dies.
You can counter Pwerfer will very cheap light vehicles, you can MAYBE counter calliope with a P4 or higher.
heck you can kill pwerfer with calli but not other way around.
And you say armor is worthless. Perhaps you play to much 4v4 where you can hide behind 3 other allies.
Survivability for all forms of artillery is huge. Especially in 1v1 and 2v2.
Pwerfer can squad wipe with the single burst he got.
Calliope will wipe but not with the first salvo (unless you're unlucky), sure it will wipe with the 3 salvos.
Pwerfer counter by light vehicule ? If you let your Pwerfer being destroy AT YOUR BASE by a light vehicule, you deserv to loose it
Armor is worthless, perhaps you should learn insta counter like loiter and strafe that one shot sherman.