I gave the first three books 4-stars (exciting but with thin characters) and half-way through A Darkling Plain was of a mind to give this 5-stars, to acknowledge the worldbuilding and story aspects that spanned all the books, even though it struggled with the same problems. (See what I did there, Philip? 'Even though it struggled' instead of 'even though it was struggling'.) The final third of the book cemented a begrudging 3-stars, it was just terrible.
Before I even talk about the story and characters, I have to mention the writing, or perhaps that should be the editing, or its lack of editing. Throughout the series, the writing was solid enough if nothing spectacular, and rarely distracting. For a while I even thought the quality had gone up in A Darkling Plain, but like the story, it seemed to collapse in the final third. Packed with action, yet filled with passive tense - these lines should be URGENT and TAUGHT but instead were weak and lazy.
"The Stalker's robes began to burn. Lightning was crawling across her calm, bronze face..."
The tone and tense was also inconsistent - making this possibly the worst written/edited of the four. It's mostly a distant third person in past tense, but every now and then we, the reader, are directly addressed, asking us to recall past stories. I rolled with it at the time, but thinking back, it was just unnecessarily jarring. Worst though, is that almost an entire chapter was in the present tense. I wracked my brain for a clever reason, because surely this was too big of an error - but if there is a reason I can't think of one. It even went into present tense for a couple pages close to the end. You'd think the finale of a best-selling series would be better edited.
The biggest sucker punch, and there'll be spoilers here, is Tom. The central character, at least for the first two books; and yet he is the same in this book as he was on the first pages of Mortal Engines all those twenty or thirty years ago. He has zero development. And then Philip Reeve makes him do something quite unforgivable and uncharacteristic: leave his daughter without so much as a hug. The reasoning itself was fairly contrived, but the act itself needn't and shouldn't have been so cold. Here we are, enjoying their conversations and five minutes later that sinking feeling that we've actually just had the final goodbye between Tom and Wren, with Tom leaving her a note to go in pursuit of Hester, giving her a kiss on the forehead as she sleeps. I'm sorry, but the Tom of this series does not do this to his daughter, not without a good hour of hugs and talk of how proud he is of her and he will fight tooth and nail to get back but if he doesn't he will always love her. It would've been hard (easy to write) on Tom, but THAT was Tom, not the man who sneaked away to never speak to his daughter again.
It was all downhill from there. I began not forgiving all the times the characters had been naive or foolish or for some reason allowed Pennyroyal, the direct reason for Tom's ill health, to continue living, or at least being a part of their travels. I'm not sure if Philip meant Pennyroyal to be a curmudgeonly bit of light entertainment, endeared to the reader - but he was not. He could've gone books ago and we wouldn't have missed him. And THAT ending - a grown man couldn't overpower an emancipated twelve-year-old with a penknife? Screw that.
And therein is the biggest problem of the whole series; amazing worldbuilding but characters that fall mercy to the plot. Things they said and did and thought, that you imagined would have a deeper revelation later on - Tom's insistence that he loves Hester, ugly, ugly, Hester, for instance, is never convincing, even with his inner monologues when he wishes she was prettier. No, 'She's ugly, but...' - it was just 'She's ugly!' Does she have no redeeming features, Tom, that you can call upon in your inner monologue? No, we'll just give her a kiss now. This makes us empathise with Hester, and she is certainly one of the most nuanced of the human characters, unable to rise out of the shadow of her father, Valentine. But even she is very single-minded.
It says something that in the final few pages you realise the most interesting and complicated character throughout was Shrike, the Stalker-robot.
I may not have enjoyed this series as much as I'd hoped, but it has been invaluable as a writer. It has really emphasised the importance of character consistency and nuance - no matter how good the world building, the characters must lead the way.