For the issue isn't even about the quality of what's put out, its just the actual content. Everything post-Reach (and even Reach in some areas to be perfectly honest) has been a poor addition in my opinion.
Starting with the Forerunner Saga, the books may be decent on their own merits, but I loathe what they added to the lore, ancient astronauts, San Shyuum/Human Alliances, the Precursors being turned into the Flood. If I had my way that would all be tossed out and forgotten about. The trend has only gotten worse with Halo 4 and presumably 5 doubling down on this part of the lore.
Beyond that there's the current sorry state of the contemporary lore. The sorry state of ONI, the incredibly annoying Spartan IV's, the most recent books and comics butchering characters like its A Song of Ice and Fire.
The lore has gone off the rails into a soft sci-fi free for all, that takes its cues from the worst parts of Mass Effect.
This so much.
As for the gameplay, I personally didn't necessarily mind H4, and I can't say H5 seems anywhere unplayable... but it's not Halo anymore. At this point it might as well be Call of Duty: Spacejocks. And while I actually do enjoy my CoDs (MW and AW at least), that does leave me with a question: why bother getting Halo anymore? Or rather: why get a 300 buck console just for Halo?
It's far from the only worthwhile sci-fi FPS in town now: Advanced Warfare was great, Wolfenstein: The New Order (which I'm getting soon) looks amazing, and I'm certain Titanfall 2 will bring a proper traditional campaign to the table as well. So, at this point, what makes contemporary Halo unique compared to the rest? What makes it worth getting a console for?
As for the lore and universe, it's just crazy at this point: to catch up between between Halo 3 and 4, you need to read through six full length novels and a comic series. That's genuinely ****** insane. To go from Halo 4's campaign to 5's, there's H4's Spartan Ops mode which at least in its original incarnation required a Gold subsciption to even view the cutscenes, another (even longer) badly drawn comic series, and another half dozen of novels of various lengths. They've completely lost it, and that's not even getting into the (significantly) lesser quality of the story now to begin with.
These people do not - or don't want to - understand what the essence of Halo is about, whether story or gameplay. Either they dislike what Halo and want to turn it into something "softer" that they like instead, they didn't care what it was to begin with and just wanted to create something of their own, or they didn't understand (lots of) parts of Halo and just filled up the gaps with the frog DNA that is Mass Effect. Either way, it leaves us with the current abomination that is Halo in name only.
And the thing is, I wouldn't have minded them creating something of their own, as I probably would've ended up liking that for what it is. Hell, I might even have become able to accept the Forerunner trilogy of books and their importance in the lore if they had made a bloody prequel trilogy of games instead. Instead they went with worth videogame storytelling sin possible, multiplied it by god knows how many pieces of pulp "literature" they have out there now, and then went on to ruin practically every aspect of the story they had.
Oh! And I almost forgot the disgusting live action mini-series they've been having as well, with the second somehow even managing to be worse than the first.