A writer for the Black Library.
He's also written several Horus Heresy books, including A Thousand Sons, the first Black Library book to hit the New York Times Bestseller list (because of the subject matter, not the author), and the famous short story The Last Church.
On the other hand, he's been responsible for some Matt Ward-level atrocities, such as claiming in his novel Vengeful Spirit that the Emperor got his powers by making a pact with the Chaos Gods (though that is, admittedly, from Horus' perspective, so...). He also wrote The Reflection Crack'd, which had Fabius Bile and friends rape Fulgrim with a pear of anguish. So fail there. Oh, and Codex: Black Templars.
McNeill's writing style is very "tell, don't show." His books tend to have characters deliver their lines in uninterrupted chunks with minimal indication of what's going on around them, and very flat emotional inflection in dialogue. His descriptions are usually pretty plain and often descend into cliche; as an example, here's some excerpts of the description of the Battle of Istvaan V on p. 466-467 of "Fulgrim”:
- “Thousands were dying every minute, the slaughter terrible to behold.”
- “Such destruction had never yet been concentrated in such a horrifically confined space...”
- “the slaughter continued unabated, on a scale never before seen...”
- “the bloodletting was a truly horrific sight...”
And on to p.471:
- "The pain was unimaginable..."
- "The battlefield of Istvaan V was a slaughterhouse of epic proportions."
- "...a conflict unparalleled in its bitterness."
- “hundreds were dying with each passing second”
He also has a bad habit of endlessly repeating awful phrases (such as "His choler rose". Yeah.) and occasionally pulling obscure ones out for use. Keeping a dictionary to hand when reading may be advisable.
On the other hand, his stories have a wealth of background information and detail, and his few character driven stories are very good as well (read Priests of Mars). And he knows how to write Mortarion - proud, clever, and ridiculously tough (didn't Chris Wraight do that first?). He even takes down a Baneblade in Vengeful Spirit, as well as catching a Fire Raptor in midair.
Priest of Mars also makes rape camps canon, just thought you would like to know (although they're heretical).
He recently announced that he had been hired as a "senior narrative writer" for League of Legends, and subsequently would not be writing as actively for Black Library as he has in the past. The response to this news on /tg/ was predictable. It's also slightly amusing, when one considers that LoL has a few shoutouts to 40k (and is very very easily amused); one character, Garen, is loosely based on an Astartes and has a Blood Angels skin.
That said he's far from being universally liked. Some Ultramarines fans view him as going too far in the other way. Making them total failures and a vibe of "Cool Smurfs don't follow the Codex" and that the lead character Uriel Ventris, becomes the biggest Mary Sue of all the Smurfs. Horus Heresy readers might criticize him for copy-pasting characters across books. Like many, many other things, this is up for debate.