A war machine is essentially a second stack in your army.
Gurani without a ballista and goblins is like a hero with one stack – it's the same as Bron with gnolls, or Shakti with one stack of troglodytes.
Gurani without a ballista, goblins, and orcs is like a hero with a 1st and 3rd stack, but without the 2nd – it's the same as Bron with gnolls and flies.

It's difficult to play with a single stack for all heroes, and this applies to Shakti, Valeska, Bron, and especially Gurnisson. But I don't see a reason to single out Gurnisson; if you're going to give a break, give it to everyone.