((No, I was referring to the Altered. I know the Nids don't really cheat beyond adapting moderately well for a species of their caliber and having an apparent shit ton of numbers, plus attacking an Imperium that can't muster a force to attack them. If the Imperium didn't have everyone else in the setting ragging on them they'd kick the shit out of the Nids.))
((They adapt really well, and there are a
crapton of them. They attack planets with literally enough "troops" that the defenders run out of bullets, plus things like quiet invisible giants (lichtors) and really frakking big ones (??), plus 'nids with all sorts of weapons. They can recycle their dead, the enemy's dead, and the local wild- and plant life into more troops. They devastate planets quickly, and their warp presence prevents interstellar communications and travel; by the time they get checked on, the planet is long dead. And their total numbers...well, their fleets are on the scale of
galaxies. The Imperium would have a huge problem dealing with Tyranids, even without everything else. They would survive for a while, but eventually they would almost certainly die if they didn't get some absolutely incredible new tech or the Emperor to wake up again.
In other words, they're a bit above average in terms of threat to the Imperium. The WH40k 'verse
sucks.))