Even though the execution could have been better in KH3, at the end of the day KH3 at least tried and did implement more mainstory elements into the Disney worlds. KH1 was still the best at it IMO because all the worlds sort of built up and/or enhanced the build to the climax. KH2's Disney worlds were the very definition of filler minus Beast's Castle, Port Royal and Space Paranoids -- all to an extent -- since those worlds just had Organization involvement, but nothing that specifically was key to the main plot or had something to do with Ansem.
I personally thought the the Disney world in KH 3 were the best handled in the series. Even though more of the worlds in 1 'mattered' in the sense of there being a princess of heart in them, I feel that only in KH3 did the Disney worlds actually each have a thematic link to the main story, which to me is more important than plot events.
What I mean is that in KH1 you go to worlds with princesses of heart and they get kidnapped and are used eventually to open a door. Save Kairi, the identities of each princess and their own world is not relevant to their role, Jasmine's role in Agrabah is not related to her role as a princess of Heart.
In KH2 the worlds generally matter in an even less individual way; Sora defeats Heartless in them. In some of them the Organisation release powerful Heartless or try to turn Disney characters into Heartless but that's generally the only connection to the main story for most worlds and it's not really tailored to each world.
In KH3, the connection of the worlds to the main story is tailored to that world, even if the connection is thematic rather than a advancement of the plot. It makes sense for Sora to be in Olympus to talk about regaining strength, foreshadowing about puppets and creation of hearts are perfect for Toy Box and San Fransokyo. The link between Vanitas' negative emotions and children's screams works perfectly for Monstropolis.
In the worlds with the new seven hearts, we actually see the princesses use their powers within their own worlds, so their identities actually have relevance to their main plot role this time.
The Carribbean's plot is a tad impenetrable but it naturally allows for discussion about the box. I absolutely love the sudden tonal shift when Luxord says 'No one knows what's in the chest we're looking for' and how, to me, it doesn't seem too contrived in context.
Even 100 Acre Wood is more interesting than previous entries. It's unusually dark at the end, with Sora worrying about losing his friends and what's happened to his heart.
I do also believe that there is foreshadowing of things to come in ReMind in most of the worlds; particularly in Toy Box and San Fransokyo but in most every world. This includes Corona and Arendelle; I don't think Sora sees flashes of Rapunzel and Anna and Elsa when returning to the Keyblade Graveyard for no reason.
Then there are the odd dèja vu moments peppered throughout all the worlds (c.f. Sleeping Realm theory), which have kept me reviewing cutscenes from every world, something I have not been motivated to do for previous games.
I am not saying you can't argue that KH3's convoluted story is bad but I do get annoyed when people say the Disney world's matter less than in previous games.