I think the main difference is in the art style and designs.
Older Disney features like Snow White, Sleeping Beauty, Alice in Wonderland & co. while having their own style are still 2D hand-drawn stuff just like classic era features of the 80s and 90s like Aladdin, Hercules, Beauty and the Beast, Lion King etc.
Stuff like Toy Story, Big Hero 6, Frozen etc. are all 3D computer animated stuff that has no relation anymore to the old classic stuff.
Star Wars on the other hand is a mix of 3D computer animated stuff and actual live action feature, so it is closest to Pirates of the Caribbean if one wants to bring up a comparison with material already featured in KH.
Like I said earlier, Episode VI is probably the part of the SW Universe that can be included as a single "window peek" with no further strings attached in KH and as Spockanort said, will probably an one-time visit only with Sora probably taking a role of more of a tag-along kid or spectator rather than being center stage in actual Star Wars events.
He'll probably work together with Han, Leia, Chewbacca and Luke, possibly flying a leased X-Wing Fighter in the actual space battle and face off against the Emperor and Vader together with Luke during the finale of the world.
The Emperor and Vader are also somewhat fitting examples themewise though for Sora to get examples of villains that are redeemable and those that aren't.
And in terms of the actual scale of evilness as well as power I'd say that Emperor Palpatine/Darth Sidious outperforms even Xehanort by vast degrees.
On the Star Wars side itself it is most likely a given that they will treat the whole Kingdom Hearts episode as completely non-canon to the Star Wars Lore itself.
Since the Star Wars fanbase/fandom is also a highly toxic one I do expect quite some shitstorms brewing if a "convuluted kiddie show" with stupid anime kids working alongside annoying Disney animals dares to touch/incorporate Star Wars.