1. The Brummbaer has a smaller aoe value than the isu and it's far more inaccurate, and at least you are able to retreat from this thing instead of getting oneshotted on retreat.
2.The Panzerwerfer is a bad joke if you compare it to the katy, on max range it's just bad and can't hit anything, the ae of it's rockets is very bad too. Gets better on closer ranges though.
3. The Sturmtiger can wipe infantry true, but normally you are able to retreat your squads from it. And it doesn't shoot out of the fog of war and it's hard to hit vehicles with it, it seems you haven't played for a few patches to me.
And the most stuff you are calling here "oneshot" machines are easy avoidable so it's more a ltp issue.
Now, back to the isu, it's not overnerfed, it's very strong vs infantry, even though i would have liked a button for HE and AP more than this micro intensive unit. Against any fast moving target it's rather bad.
The StuPa has the same AoE as the ISU. The difference is that StuPa has 160 damage per shell while ISU has 240 damage per shell. This means that the StuPa reaches the 80 damage mark faster than the ISU from damage drop off.
At 1.975m the damage is 81.12 (StuPa)
At 2.925m the damage is 80.40 (ISU)
Every man is like 2 meters tall
See the effective killing AoE?
So the killing AoE is as tall as a man in CoH2 for the StuPa while the killing AoE for ISU is one and a half man.
Btw, I actually prefer using the new ISU. At least it doesn't waste its shells randomly because there's no hold-fire.
Also, this might not be the final change. They might just give the ISU tracking for the shell like the old Sturmtiger.