So. First off. The 5 stars is because, despite some limitations (I'll touch on them as we go through this structured stream of consciousness), this game is - I'll say it now - the best version of the AGE engine I've read so far (yet the only one I've gamemastered) and as close to what I believe is possible to a representation of the Expanse book-universe (keep your canon-pants on) that we could get. There may be "better" systems out there, whatever that means in your book, but the design-fit here is nigh on perfect. One of the reasons I think so is because the the authors of the book series were involved in the design of the game (they are also roleplayers/gamers), the results of that, and how the inner workings of the AGE system are designed. So, at least by intent it is nigh on perfect, in execution (i.e. actual play) experiences will probably vary.
So, take a gander about the shop and consider other systems that could work well with this storyverse, I believe 2d20 by Modiphius could work well (somewhere between Conan/Infinity and John Carter. More crunchy than Dishonoured, but less cumbersome and fiddly than Infinity). Another system that could work is Genesys, however I believe that Genesys would perhaps take too much away from the feel of the universe and replace it with the game engine's premises of how to play the game. Still, both those system families could also quite well do what would be needed to represent the Expanse universe (Genesys would need some far better vehicle rules though).
AGE mixes something akin to the predictability and rigid framework of 5E with elements from Genesys/Star Wars and Coriolis (and 2d20). We see this primarily in that it is a pass/fail system, with levels, and level-dependent benefits (5E), the Churn mechanic (Darkness points and Doom), but also in how Fortune (renamed HP) is described and represented in the game (working similarly to Strain in Genesys/Star Wars), and of course the famous Stunt Points (SP) that is reminiscent of Advantages in Genesys/Star Wars by FFG/Edge Studio.
There was some excitement about the Churn mechanic from the designers. This GM mechanic tracks increases in potential small, big, and major interruptions into the story and narrative, bases on the successes and luck of the players. Roll well? Well, Churn increases. When the Churn tracker reaches a certain threshold, something bad happens (most likely), in increasing order of magnitude. The book here is somewhat confusing, particularly as it pertains to action encounters and the specifics of the Churn tracker, and the points that make up the Churn. At first glance it seems that it is only at certain thresholds that bad stuff can happen due to the Churn. However, when it comes to running out of ammo and jamming weapons, the rules also states that the GM can "spend Churn points" to cause this effect (hence my reference to Doom and Darkness points above). The rules are therefore somewhat ambiguous in this area. Personally, I’d suggest using them as Darkness points for smaller and personal effects, and use the threshold (10, 20, and 30 points) as group-related effects. The game also runs quite well without this mechanic, but it is also a good way for the GM to let go of control and let the story take twists and turns that are not pre-planned and scripted.
Fortune is aptly named. While you could make the short-sighted argument that “it is still hit points”, this would be to ignore the intent and the feel that this design change promotes and can create in players. Particularly with how Fortune points (FP) can be spent on dice rolls, and the necessarily attached Conditions mechanic that makes the characters in Expanse so at risk and fragile. Sure, FP is still reduced by weapon “hits” (i.e. the dice roll beats your Defence), but you can also spend the FP to improve your dice rolls. FP cannot be “healed” because it is (by definition) not real damage. There are however several ways of regaining FP during play (by spending SP and some talents), but first aid is pointless, you cannot “fix” someone’s fortune. Furthermore, some good attack rolls producing enough SP can injure, even wound (and ultimately kill) a character that still has most of their FP left. This is an elegant way of solving the issue several games have with the disconnect between hit points and critical injuries. It is, in my own not-so-humble-opinion, far superior to Modern and Fantasy AGE’s reliance on hit points. Also, conditions cannot easily be fixed during encounters, but require Interludes and doctors (live or inanimate ones) – there are however ways to ignore or postpone the negative effects (drugs for instance).
The SP system is a great addition to a pass/fail system, it is however missing the feature that is so great in Genesys/Star Wars; the fail that has positive side-effects. This is sorely missed. Also, the stunt lists are long, and the rules don’t go far enough to promote and recommend players and GMs coming up with their own stunts. As it stands, the stunt lists looks more like a shopping list you are bound to choose from, rather than making up your own. While it is good with example stunts, there is a big hole where the basic idea of what kind of effects 1 SP can cause, what effects you can “buy” for 2 SP, or 4 SP – as is present in Genesys/Star Wars when it comes to spending Advantages. This can severely limit the enjoyment of the game for more experienced gamers (if not the most veteran of us), but I can understand the desire to have a plethora of choice, if not for the newest players, then for the slightly more experienced ones. The notion of Favoured Stunts should be emphasised more strongly.
Character creation in this game is easy, and a somewhat smaller sibling of the 2d20 Life Path system. Luckily, there are no classes, only where you are from (so there is social class) and what you do/did for a living (profession). It is straight forward and gives you ample options for customisation. It also creates a fleshed-out character with a background and profession that will inspire a player to create a character in the Expanse universe. It is one of the parts I love, reminding me of Conan by Modiphius in the best of ways.
Sore teeth in the book are the Equipment and Starship chapters. The equipment chapter has a lot of fluff but is severely limited on crunch. Everything you need is there, but example weapons and armour, and other gear like drugs and medicine, is absent. The descriptions get idea of the intent across (i.e. customisation), but more examples would be great and severely improve the accessibility for new players and GMs (cybernetics and implants are also absent). The starship chapter is, on the surface, awesome. However, the system for travel and combat is condensed – and combat is, according collective wisdom missing in effectiveness, group involvement, and excitement. This last will soon (hopefully) be remedied with a new Starship book coming out, but the vanilla system will for mangy groups probably relegate starships (like in so many sci-fi and space opera games) to be a short Interlude (a nice mechanic, similar to the Fellowship phase in The One Ring) on the way to some asteroid, station, moon or planet.
When it comes to setting information, there is an assumption that the GM at least has read at least some of the novels. Still, there is a lot of information about the solar system and the various factions, we also know that the authors of the books wrote extra entries and sections for this. So, what is there, is what is known (in addition to the novels and novellas). It may be short and limited in some people’s eyes, but as a GM who loves making settings my own, this kind of open-ended writing and design is liberating and a generative. It frees me from having to stick to strict timelines and canon, to read up on specifics and avoid paradoxes and the existence of certain characters. The rulebook also covers this well enough; the choice between sticking to canon and what canon “means”. The Gamermaster chapter also has a lot of input on how to design a campaign (or series), but I'll admit that I have not read that particulalry closely, it seems (on the surface) to be similar to most of these chapters in other games, a primer for new gamermasters.
The included adventure is fun and gets you into the Expanse feel, investigation, mystery, and action - my group spent some time running through it as they are prone to distractions, but it creates a healthy scepticism in the players when it comes to entering combat and how to approach a potential dangerous and lethal situation (much needed in the Expanse). There are some discrepancies between the map and the descriptions, but this is easily overcome with some creativity.
To round up this long blurb. For a pass/fail system, Expanse (and by relation AGE) is a great narrative system which asks the GM and players to adjust their perspective on how games are structured and played in a very accessible and friendly way. It promotes cinematic and dramatic playstyles, with a lethal undertone that is certainly very suitable for a setting such as The Expanse. It is a simple system, with focus on action and the momentum players cause (and the side-effects), not in specific skill ranks and specialisations. It has an eye for detail, while keeping other aspects sweeping and general.