Yeah, it depends what you're looking for. I don't really see how the open world lacks substance, but I do find the lack of some kind of battle tower odd. Makes it feel rushed.
Definitely agree that the old rivals and groups were more interesting, but it at least felt like it had a story to tell, unlike gen 7 and 8. And the "post game" story, if we can call it that, was much more interesting than most of the stories have been.
Not really sure how I feel it stacks up against Gen 8, that'll take me a bit longer, but I wouldn't take Arceus over this. I tend to only really stack gens up based on Pokemon design and not much else, and these rank pretty high for me on initial impression. Definitely better than Gen 4,5 and 7, probably better than 3 and 8, but not as good as 1,2,6.
I will admit that this game probably won't get as much play time for me as Gen 8. The performance issues really hamper the experience, and teraraids aren't that interesting.