I'd quite like to see them as a menace for the peons who are Acolytes, but a minor nuissance for those who traverse the stars (RT's). Showcasing just how things are relative.
Reference "123.M40; The Threefold Curse" in the Calixis Sector's timeline:
Two heavily defended Battlefleet Calixis watch-stations and a sizable capital ship taskforce, lead by the grand cruiser Fire of Heaven, are destroyed by unknown assailants within a year in the outer reaches of the Hazeroth abyss, forcing a retrenchment, effectively shrinking the border of the sector and ending further military expansion in the region. The only clue to the cause is found carved on a bulkhead in the hulk of sentry-17, which reads, "The worms that walk have come for us all."
This is a reference to the Slaugth striking into the sector, probably due to the Imperium coming close to stumbling on their territories. They can threaten and destroy battlefleet taskforces. Still think they should just be a nuisance for a Rogue Trader? They are a "big bad" for the Calixis Sector, not a nuisance like an Ork Waaagh! I would dare say they are a great enough threat that they fall between a Necron Harvest fleet and a Tyranid Hive fleet.