The fact they were completely CGI aliens was inherently limiting - both in screentime and in humanising them (though of course they were an alien alien species by design) - it's unsurprising the series soon reverted to having them speak through human mouthpieces. I definitely would have liked to see more of them, but I can't see many stories beyond the ones VOY already did. They were definitely one of VOY's most memorable villain races, though, no doubt about it.
The Vaadwuar were also nice but, on the subject of underused races, I submit the Devore Imperium. Granted, "Counterpoint" is my favourite VOY episode, but these guys could have been the Cardassians of VOY. Shame about their rather bland makeup job, though.