The main caveat I have with Alola is that the region seems to happy. Too happy. I feel so bored here. There doesn't seem to be any problems. Pokemon and people live in harmony, with no apparent friction going on between them, always working together. The Kahunas, trial captains, and professor seems like to be holding your hands like
lucariomaster2 mentioned. I think its primarily because the villainous team aren't very threatening, Team Skull is nothing but misguided people, while Aether isn't really evil, it just as a crazy leader. Even if Lusamine did release the UBs in SM, they were threatening, but apparently not threatening enough until the post game where you can finally catch them and the threat is finally over. And in USM, they change Lusamine's goals so much to the point that Gladion and Lillie look like brats to the point that I could see it from Lusamine's view. And then there's Necrozma. They tried to make it the anatgonist, but it doesn't work because they always make it, " We have to help them. " for Pokemon. At least teams like Aqua, Flare, Galactic had threatening goals to the point you can't ignore them or the world is gonna end.
The second caveat is that a lot of concepts in Alola are just overdone. The concept of the Ultra Beasts and Pokemon from an Alternate universe and dimensions would have been interesting if Diamond and Pearl hadn't explored that concept with Dialga, Palkia, and Giratina. Let's not forget the Reverse World, which was our first exploration of an alternate reality, which Ultra Space pales in comparison. Not to mention several Pokemon have stated to come from other worlds, Deoxys, Kyurem, and Clefairy to name a few.
Then the whole nature and flowing with life, as well as a very watery region, was already explored with Hoenn, with already featured People living with nature ( Pacifidlog and Fortree ), Pokemon with relationships ( Seviper and Zangoose, Relicanth and Wailord ) while also restoring nature on its course ( Kyogre and Groudon. )
On an unrelated note, has it ever bothered you that Pokemon gives you 2 options for an answer ( Like when Ghetsis requests you surrender ) but regardless which choice you choose, it doesn't matter? If it doesn't impact the story what's the point of it being there?