I don't even mind the mix of high fantasy and sci-fi aesthetics. Final Fantasy as a franchise has been doing this for ages, that's nothing new.
No, what bothers me most is that the tech and aesthetics of Solution 9 are just plain ugly and poorly thought out.
They set up this dystopian world where people use souls as currency and can't remember anyone who's died, and then proceed to do little to nothing of interest with that setup, and instead just muddy the waters on established lore about how souls work in the world of FFXIV. The only part of the story that has been taking the lore of Solution 9 seriously at all is the Arcadion raid series, because the MSQ sure didn't want to explore how unstable and dangerous such a system of soul exchange would be for the stability of a nation. But because Arcadion is optional side content, they won't be able to do anything of lasting meaning or impact to the world in it anyway, so while it's the only side story in Dawntrail I've been enjoying at all, I don't expect it to actually amount to much in the end.
So while I'm all for a mixing of sci-fi and fantasy, I just want it to, y'know, be well thought out and interesting. And maybe less ugly. Or maybe...ugly in a funny way, like Allag and it's historical need to put LED light strips inside of every surface, because at least that's memorable.