Oh boy. Now you've done it.
I can't necessarily speak for Emerald, but the you can't compare Platinum to Ultra Sun and Ultra Moon in terms of differences between games, because it's completely different.
Here's only some of the stuff Platinum did (Emerald did some of this as well):
- Sped up the slow-as-hell combat from Diamond and Pearl
- Increased Pokémon diversity across the region, allowing new team combinations
- Revamped the Pokémon and movesets of Gym Leaders, Plasma Admins, and the Elite 4 and Champion to make them more difficult
- Buffed AI overall throughout the game to increase difficulty
- Altered the order in which you take on Gym Leaders to streamline mid-game
- This is a huge change. In DP you go from Eterna to Veilstone without a Gym Fight, then you go to Pastoria, and then back to Hearthome, where you've already been. In Platinum, you go from Eterna to Hearthome, then to Veilsone, and then to Pastoria, which makes a lot more sense
- Altered the story to include Giritina and the Distortion World and added some new battles, like Cyrus 3
- ADDED THE BATTLE FRONTIER, a post-game treasure trove of challenging content alongside the Battleground, which allowed you to rematch Gym Leaders, Partners, and the Rival (who actually has the highest leveled Pokémon if fought enough times, more so than Red)
Here's a full list of changes from Serebii if you want to look it over. Platinum (and Emerald) expanded upon the original two games and created a definitive experience.
What did Ultra Sun and Ultra Moon do?
- Added the Rainbow Rocket post-game arc, which takes all of an hour or two to finish
- Didn't fix any of the problems of the original game, such as the lengthy tutorials or hand-holding (which is actually upped just a little bit)
- Increased Pokémon variety somewhat across the region, but didn't really shake things up
- Altered trials to make them more difficult
- Slightly altered Pokémon and movesets of Kahunas, Rival, Elite 4, and Champion, but not nearly to the extent of Platinum and Emerald; overall difficulty is largely retained
- Altered the story and made it worse by removing Lillie and Lusamine's arc and replacing it with the Ultra Recon Squad, who just are just kind of there and don't do anything.
- Added some new Ultra Beasts and expanded the post-game quest to capture them
- Removed the Zygarde Cube sidequest and moved it to post-game
Here's a full list of changes from Bulbapedia. You'll see that the changes aren't nearly as much as Platinum, and don't alter too much of the original games.
I'm not mentioning new forms or moves as both Platinum and Ultra Sun/Moon did that (Distortion Giratina, Shamin-S, Rotom's Five Forms; Necrozma Dawn/Dusk/Light, Lycanroc Dusk).
The changes in Ultra Sun and Moon largely don't add anything on from the original Sun and Moon, especially because some content from the original games are removed. The story being altered is a tragedy because one of the strongest things Sun and Moon had going for it was it's story. Comparatively, the changes in Platinum and Emerald add on to their base games to create a new, better experience.
It's even less excusable because Game Freak already successfully pulled off a sequel game/alternate story with Black 2 and White 2, which were phenomenal re-imaginings of the Unova region, and hit most of the points that Platinum and Emerald did. Compared to it's peers, Ultra Sun and Moon are just lazy and disappointing.