OH GOD THIS MOVIE WAS SO TERRIBLE BUT THIS BOOK IS RATHER GOOD.
Warning: This will be nitpicky but I just...need to.
I'm a firm believer that a movie adaptation does not need to be faithful to the book it is based on to be a great movie. I love The Shining (I actually like the movie a lot more than the book but that's a rant for another day) however Mortal Engines manages to fail both at being a good movie and a good adaptation.
THE BEGINNING
The first like 30 minutes of the movie are almost shot for shot from the intro of the book and it's genuinely very good in both instances. London eats a small town, Hester boards, she tries to stab Valentine (in the movie she succeeds but he's fine. It's a change that I don't really understand because it doesn't actually change anything) and is chased off of London by Tom who is booted off by Valentine to cover his tracks. Tom and Hester group up because Hester is injured, they get picked up by another town who pretend to be nice and then are going to be sold as slaves.
I'm not going to go shot for shot through the changes the movie makes from the book because that'd be pointless. But I specifically wanted to recap this intro section because it's so close to the book and because where it diverges is mostly meaningless (and awful product placement.) However the part where it splits from the book is not just signposted, the script writer literally wrote into the script criticism of the book at this point in the movie. In the book, Hester and Tom remove a floor plating from their room and jump out, escaping before being sold as slaves. In the movie, they attempt this but Hester is too injured to make the jump out of the room and instead they are rescued by Anna Fang moments later. This makes sense and is a change that on the face of it I kinda like because yeah, it makes sense. Hester could barely walk at that point how did she make that jump? But the script in the film is so fucking bad that Hester may as well at this point go "I can't make the jump, i'm too injured" then turn to the camera and wink.
After this point the movie diverges in a lot of ways from the book, although it still follows mostly the same plot it just cuts out a bunch of stuff and ruins every single character then turns into Star Wars at the end because reasons.
SO
WHY THE MOVIE SUCKS
Okay, i'm going to get really nitpicky about specific changes that were for the worse that helped ruin this movie but to start with let me just summary and say. The movie sucks primarily because it's written terribly. Almost every character is reduced to shitty one liners after the intro, worst of all being Anna Fang, Valentine is reduced into generic villain, but even the characters who are similar to their book versions are just written so poorly that what worked in the book doesn't work on screen. Also Shrike's name sound so much like "Shrek" when being said out loud that it's hilarious. But overall it does stay surprisingly close to the book in plot up until the end, redacting a lot of stuff but rarely adding additional fluff. The way I'd break it down would be
First act: Most similar to the book, overall very good. If the whole movie as this good it'd be a great movie.
Second act: Starts to diverge from the book to shorten it down for the length of a film. The writing is generally very very bad here, primarily because this section focusses on Shrike and Anna Fang. The former of which is reduced to saying "Kill Hester" over and over and the latter of whom is reduced to saying shitty one liners.
Third act: It's Star Wars but bad and has almost nothing to do with the book other than the location.
Also, I'm going to focus on story and characters here but keep in your mind that after the first act the cinematography gets terrible. Like genuinely it feels like a different movie also in terms of how poorly shot and edited the whole movie was after Act 1.
THADDEUS VALENTINE
Portrayed by Hugo Weaving in the film (doing his best Sean Bean impression) and uh yeah, the changes to him are central to the changes to the whole plot and basically what makes the movie so bad.
To begin with, Valentine is somewhat mysterious to start with. We know he's the Mayor's right hand man, he boots Tom off London after he hears Hester claim Valentine killed her parents but otherwise is charismatic and generally comes off as a very affable and likeable person. He is outspoken about the class divides in London having risen up from being a scavenger to the upper echelons of society himself. Both the movie and the book do this, it's great, he's a great character.
Valentine murdered Hester Shaw's parents long before the start of the book to take a piece of a weapon called MEDUSA back to London, which is used to rebuild the weapon which they intend to use to murder lots of people and basically make London an unstoppable monster. The big change the movie makes to Valentine's character is that this is his plan. Valentine does this in secret somehow, murders the mayor, stages a coup and takes London off course towards Asia to start firing off Medusa willy nilly and kill everyone and "save London" because they'll have so much to consume.
In the book this is the Mayor's plan and Valentine is being blackmailed into doing it because he wants his daughter to grow up in high society and have a better life. This is much better for a variety of reasons but the main one being that book doesn't reveal this until much later (in fact Valentine isn't even in most of the book) and it is foreshadowed by an earlier encounter with a bunch of pirates which didn't make it into the movie (which is fine because it's rather superfluous other than this foreshadowing.)
These changes aren't inherently bad but the way they are handled in the movie it basically just results in Valentine being an incredibly charismatic but evil villain the whole way through with absolutely no depth to his character outside of that. In the book what he did to get where he is puts strain on him and his daughter's relationship in a way that builds and results in his eventual suicide by refusing to escape with Hester and Tom, despite Hester's somewhat forgiveness of him seeing how hard he is trying to save his innocent daughter.
LUKE, I AM YOUR FATHER
It's heavily implied in the book that Valentine is Hester's father and not her mother's husband. But this only comes up like once in a conversation between Valentine and his daughter, Hester never learns about it and it's kind of a non-thing. It's only really there to show that Valentine was willing to kill someone who might be his daughter in order to build this life for his definite daughter.
In the movie, this revelation is made during the climactic battle between Hester and Valentine on the back of a ship during a fucking dogfight and oh god everything about it is the worst. He literally picks her up and dangles her over the edge of the ship and almost line for line repeats the Darth Vader father revelation from Star Wars. It's insane and goddamn I hate it.
SPEAKING OF STAR WARS
The movie changes the whole final sequence so that instead of it being a struggle for survival for each of the characters and the big climactic defeat of the villain being caused by the fact he was literally meddling with powers he didn't understand (in this case, old tech than they haphazardly rebuilt and didn't know how to fix when it broke) to being a generic big budget action sequence that copies more and more stuff from Star Wars. There's literally a sequence where a bunch of fighters are taking out guns so that someone can do a trench run. The action cumulates in Tom flying into the heart of London and blowing up the engines. It's just bad and generic and literally just Star Wars. It's boring as hell. As mentioned earlier, it's also just not shot well at all and it's a mess. They do these weird awful zoom ins every time a scene takes place inside MEDUSA and it's hideous and I have no idea why it's the way it is.
Other stuff:
ANNA FANG
I've mentioned a few times in this rant that Anna Fang only speaks in one liners in the movie but I can't get across exactly how shitty it is. She's damn cool in the book and the way they translated that to the movie was basically "she has a snarky one liner for every situation."
VALENTINE'S DAUGHTER
Katherine I think is her name? I literally just finished this book an hour ago and I don't remember. One of my complaints about the movie at the time was that she is a nothing character who does basically nothing. In the book she is a nothing character who does basically nothing but she also has a pet wolf. She exists solely to give Valentine a reason for being blackmailed, which in the movie isn't even a thing so she may as well have been cut out of it entirely. She sucks in both though so whatever.
HESTER SHAW
She is constantly described as ugly and deformed after what Valentine did to her. In the movie she has a kinda cute scar but that's it. This sucks but again, whatever. It's a movie, that's how this shit rolls unfortunately. It's hard to market your movie when the protagonist looks like Two Face in TDK.