I'm thinking it's to do with the cockpit. Forgive me if I'm wrong though.
My reasoning, well I notice that for those fighters where the pilots seat in the cockpit, those are done. But for unusual designs like the Myrmidon and Ulysses where the cockpit design is "funny", they're not done.
Not sure about the Artemises though, old threads here suggest a HTL one was being made but didn't improve on detail that much. If that's the case, how about doing it the Inferno way as an unguided suggestion - stock mesh, but modified to added a cockpit? Not full HTL, but it still does wonders to make the fleet have a uniform appearance IMHO.
As for Vas fighters... I dunno. Lack of a Vas cockpit mesh? Only designs I like are the Seth and maybe the Serapis (if only for the Maxim cannon) though.

Hmm... a Seth with a Terran cockpit would look out of place...

My two cents.
