system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40K] Aside from the "main six", are there any other alien races that may pose any significant threat to the Imperium?

(OOC clarification: By main six, I'm referring to the Eldar, the Dark Eldar, the Orks, the Necrons, the Tau and the Tyranids)
gpt
The Barghesi are mentioned as being 'Hyper violent' and are quarantined in the Grendel Stars. You also have the Hruud, who are an almost constant threat on Imperial worlds, though mostly because they turn everything into garbage. 

It should be noted, however, that the Dark Eldar, Eldar, Necrons and Orks only pose a potentially significant threat. Unless the factions within each (the Kabals, Craftworlds, Dynasties or Clans) were to band together (i'm talking within their own race, not between races) they don't pose any real threat.

The Tau, on the other hand, pose no real threat even united. The only reason they've manage to make as many gains as they have so far is because the Imperium is distracted elsewhere. If they were the primary concern, they would be squashed like an insect.