Lanius is not their best general, he's the most brutal.
Their best general got thrown into the grand canyon and now leads an army of indigenous tribals against the legion' subordinates.

> When your other general is so shitty at tactics the only reason he won was because he killed everything himself and you didn't even make sure you killed him so he hates you now but it's okay because the one you have now is still better despite being such a bloodthirsty asshole he thinks he can take on the only faction in the wasteland that has fucking mininukes and a bomber plane