Is Halo hard or soft Sci-fi. It's been all over the place with the books, comics, games and TV show.
It's gradual and work dependent.
Under Bungie, it was on the harder side, especially in the novels.
343 has been a lot more ambiguous. Some works were very hard: Greg Bear's Forerunner trilogy (it's Greg Bear after all), and, I regret to say it, Karen Traviss (who when she isn't shitting on universes does produce work more on the harder side of fiction).
Then the comics have been very loose with their depiction, though I think it's artistic licensing more than anything.
As far as hardness/realism goes in the games my opinion is (from harder to softer): Halo Reach, Halo Combat Evolved, Halo 2, Halo 3, Halo 3: ODST, Halo 4, Halo Wars.