SM did a lot of things right and I wish them to expand on some new mechanics like the proper boss fights that were Totem Pokemon, as they were something I had wanted ever since I got into the Pokemon games; and Trials as a whole, they're either hit or miss imo, some are great like the Normal, Grass and Ghost ones but the Water, Fire and Electric ones, while fun, are really not that interesting mechanically. And we don't talk about the Dragon Trial.
Kahunas were an improvement from Gym Leaders too, it seemed like they exchanged quantity for quality and it shows, good god, I have never been so scared in a Nuzlocke ever since Clair and the Johto E4, Z-Moves are some scary **** that need to be dealt with quickly and their other Pokemon aren't slackers either, they all felt like actual threats.
The E4 was handled well for the most part but I felt like two Kahunas felt really samey, I know they changed one of them in USUM but they really should have gone for just Hala as he was an early challenge recontextualized, Acerola as the "familiar face, unfamiliar battle", the golf chick for the complete stranger and whiever was in USUM.
The Champion fight was one of the best though, a great step up after the disappointment that Diantha was. Kukui was a nice throwback to the scrapped Oak fight and the fact I managed to experience it unspoiled only made it even better. I like that they gave Hau some spotlight in USUM but I feel the Kukui fight works much better as you're not at all familiar with his team and the fact he uses the other starter was the best part of it all.
I also appreciate the focus on story-telling, though it did have its issues which I'll talk about now.
So, what did it do wrong? New mechanics weren't as well developed. Z-Moves are fine imo, much better than Megas for the main game.
As I said, Trials needed to be explored more, only 3 of them really felt fun and unique.
Only Totem Pokemon I need didn't quite work were Salazzle and Vikavolt as they didn't take advantage of their supports and the former's thin paper defense don't translate well into a boss fight.
And speaking of support Pokemon... **** SOS Battles, they're not fun at all, they ruin the pace of wild encounters and make catching Pokemon much harder than they should be. It'd be fine if it was a rare occurence like Horde Battles or if they were only in specific places such as Dark Grass but this is just a step down compared to those. Just bring Dark Grass back guys, Gen 5 was good.
And the story, uhm, yeah, I liked it but the whole UB thing felt forced, if it were just Nihilego, fine, but the game and promotional material set the UBs up as bosses much like Totem Pokemon so I thought there was going to be a point where you go fight them but instead the subplot is forgotten as soon as it's brought up. Type:Null and Gladion are also really underused but aside from that, I liked the more personal drama rather than "evil team bad and wants to destroy the whole world" thing that had gotten tiring especially after XY asked me to take Team Flare seriously. And yeah, Team Skull are jut pure fun, second best Evil Team imo though I still prefer Plasmaaaaaaaa.
USUM ruins every character related to Lusamine and we don't talk about it though the Ultra Necrozma battle sounds hype as a Nuzlocke enthusiast and I'd love to play it just for that.