Long story short: Imo a unit that can wipe infantry squads at 70 range does not fit the game. The Brumm was nerfed because it could do that at range 40. It was tuned down to half the range of the ISU(35). Sure it wasn't doctrinal. But the main issue with the ISU-152 is that it hardcounters Paks and Raketens before those can fight back. Brummbär can't do that - it can get off a shot with the vet 1... but that's not a permanent issue like with the ISU.
And countering it with Elefant or JT is not a good option. A unit that can only be countered by going for a doctrine is wrong design because it locks out all the other doctrines.
isu is slow and difficult to maneuver, one can easily swarm it with tank. and I think for the cost the performance is justified especially taking into account the not so good anti tank performance and long reload time