Yeah, but now let's look at KH2.
Halloween Town, Port Royal, Space Paranoids and Timeless River are four worlds that stand out because of how different they visually are from the rest. When it comes to visuals, the only world in KH3 to offer a contrast to this extent is The Caribbean.
Then in KH2 we have worlds like Pride Lands and Atlantica, which change Sora's gameplay. Another thing that KH3 seems to lack. Sora seems to have the same combat style everywhere.
That's probably because this game is giving you the option to change Sora's gameplay yourself, and usually people despise being forced into Lion and Merman forms. You can choose what keyblades you want for your style, I'm sure there will be abilities that alter combos like always, and there may be a system that lets you turn off situation commands for abilities you don't want active so how you play, Magic based, Power based, Defense based, Speed based or a mix of them, should be in your hands. Plus, if you don't want to use a situation command you don't have to.
As far as variety, if it matters more to you that in one world Sora wears a halloween costume and another he's an old timey cartoon character than it does that you're in a vast forest, a large futuristic city, a snow covered mountain side, etc then you have a strangely specific definition of variety.