I just got through this one the other night, and while I don't think I have anything new to say about how badly this world was implemented, it's not significantly worse than most of the other Disney worlds when you get right down it. Superficially, its presentation was lazy (yes, SDG are basically NPCs from Anna and Elsa's perspective), but I was able to follow it better than Tangled, if only because the Arendelle plot is rendered almost entirely inconsequential while they tried to make Rapunzel and Flynns' stories a thing and failed. But it had the same problem as every other Disney world so far, which is that SDG have no actual reason to be there, it doesn't do anything to advance their mission or clarify some necessary shift in their approach to it, and so it only serves to dissipate any sense of urgency surrounding their goal. This game just sucks at fostering mystery and building stakes, which is reflected in how poorly the worlds are integrated across the board. There's no hook.
I would say the issue of "unique areas" definitely resonates with me. KH3's worlds are clearly larger, but that isn't to their benefit because they don't provide interesting and fun gameplay opportunities (a few exceptions exist in places like Toy Box where the team really went "the extra mile"). It's 90% just run forward, run up, run forward, run up, battle some enemies, continue running, maybe the tiniest smidgeon of platforming if you're lucky and then rinse and repeat. Arendelle has a labrynthe that isn't remotely maze-like and that's it, the rest is just Land of Dragons scaled out to a wasteland that leads to a massive time sink just attempting to get from one cutscene to the next. Accessing treasure chests and hidden mickeys is often more a matter of rote luck than anything that requires thought or skillfulness; maybe you'll use air stepping or dashing in a couple of instances, but nothing that requires actual exploration or going off the beaten path-- I've actually spent time finding convoluted ways to get to places the levels themselves don't lead you and I'm always disappointed that there's never anything there to incentivize that. In and of itself that's not hugely changed from past titles, but it was much more excusable in KH2, where the worlds were small and contained and the combat was appropriately highlighted, or DDD, where Flowmotion made it a simple (and fun) task to zoom and dart through any given area (which is good, because they could also be really shallow in terms of gameplay design). KH3's Disney stuff is a definitive slog of uselessly massive battle arenas and the same cliff faces copy pasted world after world. That's not to say they aren't gorgeous, but they could have been the size of KH1's worlds, retained the same amount of content, and been just as pretty while placing much less demand on the player's patience. And then there could have been more worlds overall, including original ones which got the shaft.
