So, I recently cleared the main story of Ultra Moon and I can say...it's not as bad as I gave it flak for. Yes, it's a retread in much the same way B2/W2 were, though I have more praise for it than those games which only brought the generic structure of the franchise back into sway. USUM Lusamilf is a far more understandable and justified evil team leader than her original self, as well as any evil team leader for that matter, as her goals were in wanting to protect Alola from Necrozma rather than getting her mind altered Lovecraftian style and wanting to merge with an interdimensional jellyfish. Hau was also better due to not being quite as overly happy 24/7 and being treat as more of a genuine challenge in the Champion battle (even if a step down from Kukui's challenging team). There were some things that were a step down, such as the Elite 4's team not being that challenging with the removal of Olivia's Golem, Acerola's Sableye and Kahili's Skarmory, though most of the Totem Pokemon were more challenging and Mina's trial being added was a nice choice. Ultra Necrozma's difficulty spike in it's level and stat boosts made Nihilego Lusamilf from S/M look like a Route 1 trainer, providing a pretty good challenge.
Gameplay-wise, Roto powers were also a cool new feature as it brought back the easiness O-Powers provided, yet limited their availability and made access to them far less in your face than O-Powers were in Gen 6 for those who preferred a challenge. Being able to earn Totem Pokemon was nice as well, though Mantine Surfing was worthless.
Overall, I'd say they were an improvement on Sun and Moon and cement Gen 7 as being decent, at least above Gens 1 and 3 as well as maybe 4 if removing HG/SS.