It also divides the pocket-book: many players won't invest in their current armies and instead throw money away on this sub-game. It takes everything that was bad about Apocalypse and multiplies it by ten.
Just like Apocalypse, the Horus Heresy will be a game of one upmanship, as rich players see exactly who's Heresy is that much bigger.
When Apocalypse came out it quickly made my local meta unbearable. As someone who doesn't enjoy large games it left me out in the cold half the time as everyone rushed to play 10 hour games that they never finished.