If I'm being honest this arc is, quite frankly, a mess.
As far as characters goes, I like them all. Sun is alright, Moon became one of my favorites DH pretty quickly, and I like how all the games characters are adapted, specially Lusamine, whose spe version has to be my favorite incarnation of her. Making her even feel even more insane and creepy while at the same time doing away with most of her cartoon villain traits was a great call. However I feel like almost all of those characters are pretty underused, even Moon and Sun in a sense.
But the story is, basically, non-existent. When I reviewed ORAS here, I said it was 'much ado about nothing'. This is the same but even worse, because at least for ORAS everything lead to something, even if it was underwhelming. Seriously, what is this arc even about? The only plot thread that properly develops are the UBs invading Alola, but there's 2 massive issues with that:
1) That's not that interesting of a story. It was the character work that drove the SM/USUM games, not the plot, and there's very little of that in the Pokespe version.
2) There's bunch of a set-up for unrelated stuff that leads absolutely nowhere.
To explain this in more detail. I don't know if you've noticed, but this arc is mostly organized in a way to have mostly self-contained stories in each volume, each corresponding to an island (with the last two being for Ultra Space and the final battle). So let's start from the beginning. It starts introducing the setting and establishes the major characters with that tournament in Mele'Mele. Fair enoug. Then in Akala Moon investigates Ultra Beasts while Sun does the trial. If the dynamic of the entire arc was like that it'd be much better. But then in Ula'Ula weird narrative things start happening.
While Sun goes with Lillie to do another trial Moon ends up in Aether paradise, where...nothing happens. It leads absolutely nowhere. Then the 'mini-arc' culminates in a climatic battle against Guzma and his UBs, with Moon joining up at the last minutes and everything ultimately ends with Guzma getting kidnapped by UBs, which also, ends up going nowhere since all we get from that is set-up for Motherbeast and we don't see anything about Guzma's motivations or his relationship with Lusamine.
If Moon getting removed from the story while Sun does a sidequest, followed by an unnecessarily long battle and Moon showing up at the very end sounds familiar, that's because it happens another two times: In Poni and in the final battle.
But those are far from the only things that go nowhere. Moon's goal of saving her Piplup is forgotten almost immediately, while Sun's money-collecting also gets forgotten shortly after the dramatic reveal about the island, and in the end everything is just solved off-screen?. One could argue that speaks of their characters that they're willing to put their goals aside for the sake of the region, but honestly that's not really focused on at all, and they don't even help all that much. In the end is Lillie that deals with Lusamine and Gladion saves Moon from Faba. And the only reason they are the ones to defeat Necrozma is because the legendaries chose them. So in the end, it seems like the story is still about the aether family, but for some reason Sun and Moon are the main characters we're following, despite being only tangentially related to the whole thing. They end up feeling somewhat like those self-insert OCs fanfic writers sometimes force into the main cast. And even though the relationship between Lusamine and her kids should be one of the main driving forces of the arc, it still doesn't get proper resolution. In fact, almost nothing does. That's not even a bad ending. It's straight-up a non-ending.
Then there's also more things that have no pay-off like Plumeria disguising herself as Moon and Hau's character arc. He was supposed to have one, right? And wasn't Sun supposed to be doing the island challenge?
And then there's the dozen or so battles again Ultra Beasts that often take several chapters. This is not Space Invaders. 'We gotta beat the aliens' isn't gonna cut it as a plot. In fact I'd say this arc's biggest flaw is having too many action scenes and too little character development.
XY was almost as short as SM but way more solid. And that's because its story-telling was much more focused. It had one single storyline that we were always following from beginning to end. For some reason, SMUSUM decided to do the opposite: set-up a bunch of stuff without committing to any of it. And the result is...nothing. I honestly see no story here. Just a bunch of stuff that happened.
I don't really dislike it because the characters are enjoyable and fun to watch, but I can say it's definitely my least favorite arc as of now. The volumes are gonna need to have some major changes to put it on the same level as the rest of 'main' arcs, because it's honestly not even on the same league.