My complaint about Yozakura remains the same, Gondaira simply has no clue where to take the series. Trying to introduce a badass new rival out of the blue for a serious arc simply doesn't work when the series already has over a dozen characters, the majority of which it has done nothing with. This sudden tonal shift feels less like a deliberate decision (like it was for Gintama and SKET) and just another case of trying recoup readers who're complaining about the manga being boring.
Also, my opinion on Agravity goes as follows, I don't even know what to think of it. Simple as that.
Yeah, the "Japan doesn't like Sci-Fi" comment is both nonsense, and not even what Kishimoto said. He said that Sci-fi has a tendency of failing in Jump because its "complexity" means its easy for readers to quickly lose interest and gravitate to other series.
Anyway, the primary issue of S8 for JP readers, as has been said in numerous reviews, is the sheer amount of exposition and intricate world-building making the story too difficult to connect with. Kishimoto's story and character-writing aren't strong enough to compensate, and the result is the series being not able to grab them. It's basically too much Kishi's own thing.