It would also encourage combined arms.
No, it would FORCE combined arms and that is bad. I'm not arguing that the ISU or jagdtiger is totally fine as it is, that would be absurd. I'm simply suggesting that limiting the amount of them that can be called in doesn't really solve anything as far as diversifying the metagame. It doesn't change the fact that they're overpowered either.
So I think they need to have their performance adjusted such that calling them in even multiple times is viable in the right situations but not overpowered.