The GMC can WTF own infantry pretty well, and I've seen it tear my squads apart at range.
The Stummel really really really really blows though. I made a new TH company just to try one out. Even against huge Rifle/Ranger blobs the Stummel just can't get any kills, or even deal any real damage.
I remember a game played vs some Infantry spam on Neuville once. I called the Stummel on early, and for nearly the entire 45 minute match, it got a total of 2 kills, despite having fired near endlessly into some marauding infantry blobs.
I seems to lack the AoE necessary to deal damage to entire squads, nor the damage to get kills off of direct hits, the range to safely engage squads with Bazooka or AT Rifles, or even the damage modifiers to make it useful vs garrisoned troops. The thing just blows.
Some of what you say isn't the full picture.
Strummel will kill rangers/AB on direct hits and up to 1 meter away from target, and excluding airborne in light(sometimes will regardless), and excluding both in heavy, seeing that it deals 87.5 dmg.
