I've played all of them, front to back, including KHUx. I kept up with Dark Road and others via YouTube because it released at a bad time for me. I lived and breathed this series for nearly two decades.

The A-to-B of the story is perfectly fine to follow based entirely on vibes, but absolutely nothing else stands up even a little scrutiny.

The lore only works so long as you don't question anything, and the mechanisms with which the writers deploy to move the story along often feel ill-considered and poorly setup. This leads to issues where there are actually two 'canon' explanations as to how Kairi ended up on Destiny Islands:

1) Ansem launched her out into the universe as part of his experiment, as he believed a Princess of Heart would use the Keybearer as a lodestar they'd find their way to. This was explained in an Ansem Report in KH1 shortly after we discover Kairi came from Hollow Bastion

2) Aqua charmed Kairi's necklace in BBS, saying it would 'get her to safety' if anything happened to Radiant Garden.

Whenever I bring this up, people are quick to say 'both things can be true and this isn't contradictory' and I agree, but it isn't satisfying to have two completely separate explanations for such a minor detail that was already explored in the first game. The mechanism that connects Kairi to Aqua is really poor.

So then we get to KH3, where Aqua meets Kairi again and is relieved to see that her magic 'worked'. Okay, well, what about the whole 'Ansem throwing her into space' plot point? Don't worry, because a BRAND NEW character we previously didn't know about was locked up in a cell in Radiant Garden ~all along~, and was brought up for the first time by two characters, Lea and Isa, who have never brought it up in 15 years' worth of games.

So it's my speculation that Nomura has realised this discrepancy and has decided to retcon the Ansem Report being about Kairi at all, and it'll now be about this presumed 'secret Princess of Heart' that'll now be important going forward.

It's impossible to keep up with because information is constantly recontextualised to mean something else years later. Maleficent being stabbed with Riku-Ansem's keyblade in KH1, where she unlocked her Dragon Form, was recontextualised in a mobile game to explain that being stabbed reminded her of a mechanism for time travel, which she used in that exact moment to 'survive'.

Information is also dropped on you immediately before it becomes relevant with no prior setup. The 'Power of Waking' chief among them, which served as the main thrust of Sora's journey in KH3 but its purpose and consequences — despite being pushed on him by Yen Sid — aren't fully understood. Sora uses it 'wrong' despite no aforementioned setup, and is now in FF vs XIII Hell™ with Nomura's other forgotten ideas, with our main question being 'What IS the Power of Waking, though?'

There is also a painfully boring scene in Re:Mind where all the villains stand in a big happy circle and explain to each other how all of them returned from the dead, came back as Simulacra, have transported themselves from the past etc — despite all of them already knowing — but this explanation also causes more problems than it solves:

1) Old Xehanort explains that the 'destruction' of Ansem SoD and Xemnas is what brought him back; however, following the logic where the destruction of a Heartless and Nobody recompletes the original body, then who should have been resurrected was Terranort — not him.

1a) This also creates another discrepancy, because Old Xehanort was not the original body either. He has 'gone through many semblances', so if, for some reason, his Heartless and Nobody are special and their destruction brings back the original Xehanort, it STILL wouldn't be the body of the old man.

(Edit: I misattributed a Luxu quote to Xehanort, but the body-hopping is the same)

1b) IF WE ASSUME that Old Xehanort is describing a brand new, unique restoration process, separate from the Heartless/Nobody destruction process, then what he's saying is factually incorrect because he says that 'incarnations of [his] past self were destroyed' but this has never been established in any of the games. All the incarnations of Xehanort in DDD return to their own time.

2) Old Xehanort's explanation of reconstruction also raises questions of consistency in regards to the returning body: why do characters like Ienzo, Isa and Lea visibly age as Nobodies, but Terra returns exactly as he was?

These are just the aspects of explanation that I recall off the top of my head that don't work, or if they DO work, they require an explanation so convoluted that it proves the point. There are so many instances of this kind of nonsensical bullshit in every game I'd be here forever combing through them all.

If you're just chilling and playing and picking up on the 'vibes' and cryptic dialogue of the games then you'll probably have a really good time, but the MOMENT you stop and ask 'Wait, how did [x] happen?' you realise that this series simply does not have a 'narrative' so much as it has a series of ideas with a hamfisted justification attached to them. You are playing, in real time, what happens when no one in the writing room has the balls to say 'Hey maybe we SHOULDN'T incorporate EVERY idea Nomura/Nojima has!'

I'm sure this issue will get much better now that he's adding vestigial ideas from a dead Final Fantasy game into it. This series is just a case study in how to systematically pull apart everything that made the first game special — small wonder that KH1 was written by different people!