The ISG isn't able to damage buildings very much, and it's only effective when used as a direct fire unit when the enemy is in direct LOS, making it very ineffective as a indirect fire unit.
Sturms are good against some houses depending on the window placement as well as who loses more models first, a full 4 model Sturmsquad can normally pull it off if it's lucky, but many times you will just lose a model and then lose the DPS race.
There are tons of maps were buildings are unavoidable, and the Flak HT damages houses far, far faster than a Puma will, telling a person to get a Puma to counter buildings is insane.
Shreks barely damage houses at all, as do all forms of infantry based AT thanks to the garrison revamp after WFA.
Again, pure bullshit about game mechanic.
GrW43 is able to kill Rifle squad in wooden building in 1:14 (building collapses - not killing squad inside - 2 models died with collapse).
ISG can do same thing in 1:28. 2 Models left which died with collapse.
RNG involved cause depends where shells will land but still, bullshit decetor is needed