343's Frank O'Connor explains what went wrong with MCC and what's next for the game.

Halo: The Master Chief Collection was an ambiguous and complex project Microsoft and 343 Industries set to develop. 6 engines running from Original Xbox and Xbox 360 on a single package for Xbox One sounded astonishing on paper, but when the game came out on November 11th, 2014 everything crumbled. Bugs upon bugs, network issues, saving issues, long matchmaking times were plaguing this package. 343 promised to fix the worst issues – which they did – but still left the game with many problems to this day. Frank O’Connor took to Halo subreddit just over an hour ago to explain what went wrong with this game. We will give a short version at the end of the news if you’d like to read it.
“We’re explaining some of the issues in more depth than we have before, in part because we now have the resources, OS and capability to make meaningful changes” Frankie starts explaining, ” We’re trying to be as transparent as possible, but there are loads of proprietary things we can’t talk about at a granular level. ”
“On Wednesday of this week, we announced that we’re both fixing MCC and working on enhancements for the Scorpio (Xbox One X) version of the game, but I should be clear here, that in terms of chicken/egg scenarios, fixing the existing “vanilla” Xbox One MCC was the Chicken that laid the Xbox One X enhanced version egg. Without the ability and opportunity to reconfigure and fix this thing, we wouldn’t touch an Xbox One update. But a series of changes to the Xbox architecture, some of them related to Xbox One X – and others just a series of ongoing improvements to the OS and back end networking systems, have cracked open an opportunity we’ve wanted to seize for many, many months now. So to be super clear, these fixes will apply to both regular Xbox One version and the Xbox One X enhancements.”
He continues to say, “We’re also getting a lot of help from the (wholly separate) Halo 5 team, who created a much more robust system for the launch of that game and continue to make improvements to their networking model. From a personal perspective, the MCC launch was one of my lowest ebbs, professionally. Every angry mail I received, I took to heart. I felt like I had personally let our fans down. I have not spent a single day since the night the game fell down in matchmaking where I didn’t think about it. The hardest messages to deal with were the ones driven by disbelief. “How could you not know that matchmaking was going to break?” – fundamentally it was because we were testing it in an environment that we had set up incorrectly and with some (as we discovered later) faulty assumptions. And unlike some of our other normal testing cycles, we weren’t testing for gameplay balance and stuff that the original releases already contained so our test process was radically different, and we made mistakes in some of the scenarios we asked for.”

“We had, with the best intentions, created a massive and ambitious project that almost read like a Halo fan’s wishlist. As a player, I was incredibly excited. And as an employee, I was proud of the work and effort the team had poured into making this thing so big.
It initially started as a conversation about making a Halo Anniversary 2 – we thought about simply replicating what we’d done with the first Halo: Combat Evolved Anniversary, a polished update with some cool new graphics and features. But we kept talking about it – and the conversation inevitably led to the “problem” of a franchise existing over multiple generations of hardware. This was built for Xbox One – and prior games were spread across 360 and OG Xbox. So we figured, why not finally put the whole Chief saga on one console? We wanted everyone to be able to enjoy his entire story.
And so the project ballooned in scope and scale and ambition. We threw a ton of resources behind it internally and worked with some trusted partners. In our matchmaking testing we were seeing results that ultimately weren’t reflective of the real retail environment, and our test sessions never got to the kind of scale where we’d see some of the looping issues I’ll describe below. So we genuinely didn’t know until the day it released, how bad the matchmaking in particular was going to get. I’m not going to ignore the other bugs, they were real, and important, but the way the UI and matchmaking protocols interacted with each other exacerbated many of the smaller items and amplified a couple of them in unpredictable ways.”
Frankie ends up the blog simplifying – “It may sound simplistic, but MCC was essentially six pretty different game engines strapped together and interlinked with highly complex and highly delicate new systems. With Xbox One X on the horizon, it was obvious that we could simultaneously update the game to take advantage of the new hardware for folks that have it and use that as an opportunity to finally rearchitect and update some of the foundational issues and networking/matchmaking methods. I’m going to follow up next year after we have better detail on the fixes and the Xbox One X update, to follow through with an even more detailed technical breakdown of what broke, why and how we fixed it. That’s what we owe you – that and a game we can both finally be satisfied with.”
In short, MCC was a massive and difficult project for 343 Industries and all studios that helped make this game. Putting together six engines from different console generations in one package was too much and everything started to fall apart. Internally, the game was working as intended and this was something an Xbox employee told me late 2014 with just over 5,000 people testing the game within Microsoft, 343 and other studios. In retail environment with, literally, millions of players, it was a kind of pressure the game was never put into unless 343 made a beta so they could have foreseen these issues early on. Frankie will give a follow-up when an update releases next year for MCC and we should have even more information about what went wrong with MCC.