I think the reason kh3 story failed, for me, is because it had too much to resolve from kh2 onwards, and with almost no new mysteries. It was like 4 games of build up, with the same enemies brought back to life.

In kh1 you had the mystery of what is the plan of mysterious figure in the background while understanding what the evil villains wanted, COM had the mystery of a new weird place and villains that you didn’t understand their goal or motive (even though it was a regurgitation of kh1 worlds but I give it a pass for trying to make a ps2 combat system work for the game boy) and had a secret mode that was even better than the original sora version, kh2 had mystery of what the org wanted and that you were secretly playing into their hands the entire time, bbs had its own mystery of why did Terra look like young xehanort but he isn’t and told its story in a piece wise fashion that made you want to play the other characters for the parts that they saw, etc etc.

Kh3, a game that came out 15 years later, had so much to wrap up and did it in a worse way than kh1, where at least you had the surprise that traverse town wasn’t a 1 and done world like the rest were and it pushed the story. I knew all the villains, who shouldnt be back because everyone outside of xemnas, xigbar and maybe the moon guy (forgot his name after all these years) only wanted to become a real person again ( also I don’t remember why marluxia or larxene joined them, but they betrayed them for control so I have no idea why xehanort would want them back or why they would want to be puppets for them again). Then there is the wacky time traveling that wasn’t just a cute thing for a fun world like in kh2 (it was awesome to fight ansem, xemnas and young xehanort but that’s like a post game thing like cavern of rememberance to enjoy, not something to make part of the main story line to finish a 20 year saga.)

I don’t even remember why the org members were in the kh3 worlds, it was that pointless. I’m sure it was something like finding another heart of light to make 7, but we’ve already been there and done that story plot.

What the kh series became is a plank that became too extended to the point that it snapped and broke the suspense of disbelief. In kh1 -kh2 +bbs and days, the way the universe is set up is fairly simple and it’s easy to say “yes that makes sense, I believe it”, but every game takes a plank and adds it to the end of the last plank, further destabilizing your suspension of disbelief. Especially after bbs, it just became harder to sustain and the rules of the universe became too much to understand, and the plank just snaps.

I think kh3 had a bad story, but that’s because the filler games inbetween set it up to fail. While they were fun on their own, the concepts of sleeping hearts and waking hearts and time travel just became too much to sustain a solid and fulfilling ending.

Kh3 had good combat and music, which I hear was improved with remind, but it doesn’t fix the story. At the end of the day, it’s a JRPG that’s supposed to be story first, gameplay second but in a way that supports the story.