I would say the main issues the game faces would be as follows.
1.) Obviously the position of the Xbox One lowers the amount of sales it can possibly have, so that's an initial headwind.
2.) However, I think the bigger issue is how much the market has changed since 2012. First party games in general are way less relevant as third parties have taken over through economies of scale. In the same ~1 month period, each of the three biggest third party publishers is releasing a major competitive online FPS with astronomical sales expectations. Black Ops 3 is 20+ million, Battlefront is 13+ million, and Rainbow Six is 7+ million, and they all have development, outreach, and promotional investment to match. To note, by development, I don't necessarily mean cost (though that can certainly apply), but also staff who have a good feel for openings and trends the market. I'd view Rainbow Six as the shakiest of those, but they are trying to find an opening in the market by releasing a Counter-Strike like game on consoles.
3.) Halo hasn't been seen as a seminal franchise for a while. Halo 5's reception seems better than Halo 4's, but since it's not seen as the game from which other shooters derive, it's harder to generate an astronomically high attach rate result like Halo traditionally has. Instead, it's now simply another high quality shooter you can buy from all the other good options this Fall.
