I think the infernals could do with new art for warrior/scout/settler/worker/archer.
They're Demons, not Undead. although the two are similar in alignment and aims, they are still very different groups of creatures.
I'd suggest making one or two generic demon models, and reusing for them all, just changing weapons/tools.
The undead models they currently have ARE very good, they just don't fit with the race, especially given that those units have the Demon promotion, and upgrade into proper demons. The current models are better off used by the Scions of Patria and Legions of D'tesh