The Brummbar is no StuH. In terms of function it serves the same role, but both vehicles utilize weapons of different calibers, ranges and damage, a different chassis, different hp etc.
StuH 42 is "only" a StuG with 400 hp and a 105mm howitzer that deals 100 dmg every 6-7 seconds. (oh, and since the 4cp-change in 2.602 it's actually considered slightly OP in vCoH)
The Brummbär used to have a pIV chassis with 700 hp, a better armourtype than a pIV and used a 150 dmg howitzer every 5-6 seconds.
Because of the higher base damage it used to 1 hit kill entire squads. A single Brummbär could dominate the game easily, especially since its well armoured and can take some hits from AT guns. Just like the Sturmtiger. The only chance to actually kill it are soviet tanks, but they're also well countered by Army Elite's Tiger tanks. So overall, the Brummbär is uncounterable and army elite OP.
Best AI, best infantry and best tanks. That's why it was nerfed and the Sturmtiger removed from Army Elite.
