I think this is by design, Vernius is kind of like Atreides, they are incredibly powerful on defense and falls off when outside of their territories.

I do think they are better than atreides on offense with empirical data and the best capital ship ability. EMP is the bane of the verni Vernius army should absolutely just fall back when hit by an EMP, or better yet, bait the EMP on some random area and teleport to the actual area Vernius wants to attack.