SPOILERS. THERE WILL BE MANY SPOILERS. DO NOT READ IF YOUR GM IS PLANNING TO RUN THIS
TL;DR: The book is overall pretty good. It's not a literary masterpiece by any stretch of the imagination, but the chapters that work, work really well. There are also chapters that are awful, and you should just straight-up ignore. There are also some very questionable editing decisions that mean you have to cross-reference things across both ends of the book, but this is something that can be overcome by taking notes and assembling a timeline yourself. This started as a review, but ended up being more of a GM guide on how to use the book. I hope nobody minds terribly.
Caveats: I am not going to be doing a deep lore analysis here, I'm just taking everything at face value and talking about the usefulness of the material to a gamemaster. I would classify my knowledge of Shadowrun lore as pretty decent. I'm not one of those amazing madlads that know every character and historical event in Shadowrun lore, but I know enough to get by and give my players an immersive experience and get the "feel" of Shadowrun right.
I'm gonna be doing this chapter by chapter, and giving my thoughts and practical tips on each.
Snafu
As an introductory short story acting as a springboard to understanding the next chapter, it's not terrible. Although the Starship Troopers style "They're everywhere! Oh god the bugs!" is very hammed up and the milspeak is a bit wrong and jarring, it gets the message across. This was a useful read to get the feel of Ravenheart and her relationship to Ares, and Damien Knight's goals in Detroit. It also sets up the underground tunnels that the bugs are using to ambush Ares personnel quite well.
Also, just as a small note before going into the next chapter, I've seen quite a few people accuse this pair of chapters of being Chicago Bug City 2: Electric Boogaloo. I disagree. There's enough new stuff and different twists on it that it really does stand on its own. It builds off the events of Chicago quite naturally (from the Ares point of view at least).
Detroit Rupture
Basic rundown: Ares has been experimenting with possessing insect spirits with human souls. They're doing this to try and control some of the bugs, as a weapon against other bugs (Alpha Merges). They attract a whole bunch of bugs to Detroit via unknown methods, and then of course the controlled bugs go rogue and start killing everyone (who had the bright idea of using psychopaths and/or murderers to pilot those bugs?!?). Lots of fighting, a few b-plots (like Ravenheart's renegade Firewatch crew and the 61st Independent Rangers), and then Damien Knight gets murdered by mysterious space laser. Arthur Vogel blows up most of the remaining board members in a false flag and becomes the new head of Ares. Official coverup story is that anti-Ares terrorists wrecked the city and Ares bravely fought them off.
So... where to start. This was the hardest chapter to pull all the dates out of and assemble into a chronological order, mainly because a lot of it split between this and the "Detroit Now" chapter. Make sure you read that chapter after this one, then go back to "Blackout" (you'll see later why you should skip over "Ghost Army" entirely).
The characterizations of Marv & Co. were pretty cool, and I used Platinum Trollgirls as the players' base of operations. There's quite a lot of interesting RP moments, clashes of worldviews and a general "Fellowship of the Ring" vibe you can get with shadowrunners, legal mercs and ex-corp military under one roof. Over the course of their time in Detroit, with player help Ravenheart made her own little army of Ares Firewatch defectors in the basement, and my players had a lot of fun, sad, and weird interactions with various ex-Ares military folk.
I kinda went hard on the idea of the split between loyalty to Ares and loyalty to Ravenheart for the ex-Ares soldiers, which made every scene where they had to work with the party against Ares very tense. I even made one a full-on Ares spy reporting to Damien about Ravenheart's whereabouts. Ravenheart is now a pretty important and integral contact for my players, and I'm glad I managed to make it work. The 61st were harder to use for me... they ended up feeling more like hired muscle, which I guess they are? I could have put more work into them, but they worked in the role.
I'm a big fan of the "This Just in: Motor City Mayhem!" section of this chapter, and actually copied it, formatted it, and had them find it on a corpse while doing a patrol of the area. It's a short transcript of a reporter in over her head trying to report from the front lines who doesn't make it (in my version at least). Was a nice humanizing moment for the party that underlined the fact that not everyone left in the city is an experienced fighter. I ended up editing the final entry of the first half because I didn't want to add any more b-plots, but if you want to expand the ghoul vs. ghoul stuff into a full run you definitely could. Of course, I didn't use the second half later in the chapter about the finale, but you could easily have her as an NPC, and/or have her show up at the finale.
I'm going to skip over a lot of detail about the Apex teams, the Alpha Merges, how Latvian Gambit was supposed to work vs how it played out, but you can read the book yourself if you want the details. Long story short, it's pretty useful stuff that you can sprinkle in as clues to your players as to what's going on at a big picture level while they're scrambling about trying to survive.
The 4-way standoff that happens between the bugs, Ares, Ravenhearts' irregulars and the Alphas is an amazing opportunity to do any b-plots or wrap-ups you want to do before the big finale. I used it to let them have a close encounter with a wounded Alpha to try and give them a few more clues as to how they worked and what they are (obviously they just shot the thing without talking to it because "bug scary, shoot scary bug" :P) and some helping out of the locals so they could feel a bit more heroic than your standard shadowrunners. I also had them set up a signal booster on top of a building to report to their fixer, and had an Ares-affiliated shadowrunner team try and murder them. It's always nice to reinforce the idea that your players are not the only shadowrunners in the world when possible and practicable.
You could do the finale a bunch of different ways: having the players defend the Platinum Trollgirls, having them at Ares Tower, off doing some spec-ops stuff to track down Otto Hendricks (big boss insect shaman) and assassinating him, but I chose to have them at Ares tower, with Ravenheart swooping in to save the day and saving the lives of people who think of her as a traitor. Was a fun moment, and the near-misses on blue-on-blue as the Ares soldiers eventually go "Fuck it" and ignore orders to shoot at Ravenheart to have the help vs the bugs was fun for all.
Whichever way you do it, make sure you really sell the gravity and the insanity of what happens to the tower, and the fact that this mysterious space laser attack just killed one of the most powerful metahumans on the planet.
Ghost Army
After starting with probably the best chapter, time to go to the worst. Oh boy do I think this chapter is just hot stinking trash. The book would have been improved a lot if they had just deleted the whole thing. For obvious reasons, I did not use this chapter and I recommend everyone else pretends it doesn't exist.
Sandwiched in-between a huge city-wide battle between 4 sides that ends in a climactic event that will be felt by one of the big ten, and a string of mysterious EMP attacks that leave millions without power, water and making food a problem long-term, is a bunch of no-name soldiers going missing due to a paracritter. That's the whole chapter in a nutshell. There's a paracritter. And it killed some soldiers. That's the whole thing. They describe this as a UCAS Corps, which if it's anything like the current US Corps, means tens of thousands of troops. The transcription is from the point of view of the poor schmuck sent to find out what happened to them, as his recon squad is slowly whittled down. Now, I think it's a perfectly reasonable spooky run setup, but the difference between a recon squad mysteriously going missing and up to 80,000 soldiers going missing is pretty big. This whole thing makes no sense, doesn't connect at all to anything around it (with the exception of some handwavy "3rd corps went missing" single sentence in the next chapter), and is just plain stupid.
This might still be usable if they went into any detail over what the paracritter is and how it deleted tens of thousands of soldiers from existence without a single one managing to radio in to say that something is sus. It doesn't. It just basically goes "lol superpowerful paracritter, figure out how it works and what it does yourself". It's absolutely stupid no matter which way you look at it.
I hit the character limit for posts, continuing in a comment below.
At a conceptual level, it makes 100% sense that companies are moving away from bespoke engines and towards a standardized engine. This is something you see in many different industries. Spending a whole bunch of money and effort when 95% of your requirements are the same as everyone else is a bit silly.
However, you're completely right that there's the same danger as in other industries where the 'boring software infrastructure' gets standardized and outsourced: a niche monopoly able to exploit their position. What tends to happen in the long run is an open source project for that infrastructure run by an NGO with the industry members (in this case AAA game companies) on the board of directors.