The mutated humans had a fatal weakness - they couldn't stuff their giant bodies into vehicles. Even if they always arrogantly referred to themselves as moving armor, they were just trying to make the best out of a bad situation.
If it wasn't for the Dusk Church modifying trucks for them, they wouldn't even be able to operate vehicles.
In the early days of warfare with Fishbone base, they learned how to install cannons onto their trucks. But faced with the Tiger II, their crude "high mobility anti-tank cannons" were a joke.
The most common anti-tank cannon used on the wasteland was Type-99, produced by deconstructing the primary cannon used on the Type-99 light tank. The Type-99 shells couldn't penetrate the thick front armor of the Tiger II, however. At most, it could break the belt and destroy the observer hole, but one round of Tiger II's 5 kg bombs could break the cannon along with the truck into pieces.
