It's a tie between Ivalice and NieR from what I saw, and for two different reasons:
Nier's story was hated because it became clear after a while that it was a glorified crossover event and you needed to know the Drakengard/NieR plot to understand what was going on. It ended without proper answers or a satisfying blend of the raid into FF14's lore. Oh and the two lalafell NPCs were beyond irritating, with their ending leaving some people feel like the 'feels factor' they were going for was undeserved.
Ivalice's story from minute one was hated because it relied heavily on "Tell, don't Show", blitzing through a lot of important plot details that only made sense if you knew of both Final Fantasy Tactics and Final Fantasy 12. Then the characters it introduced were terrible, with Montblanc and Present-day Ramza garnering hate for being insufferable. Then, some raid fights were way too hard for the majority of the playerbase, with notorious fights such as Hasbrown, QUICK MAFFS, "Don't stand on the circles during Dark Geas" Rofocale, HP Sponge Yiazmat, Thunder God Cid... Eventually it also became hated due to BROWN. Bozja being another round of "Tell, don't Show", it being another grind fest with an oppressive environment and forcing you to finish the Ivalice raids and run them like 10 times made it all worse.
I liked the raids due to their presentation and gameplay. I loved the fights not holding your hands and outright having the balls to tell this playerbase to just get good. But the stories were not the best, and eventually the playerbase's cries of foul won out, with Ivalice getting nerfs and item level alleviating some of the issues with NieR. Either way, people still hate on them to this day for being too hard/HP sponges.
...
Apart from that, I agree with your previous post. People hate on these and don't want fights like these, but at the same time so many people to this day, Mentors included, would wipe at Phlegethon's flare and Angra Mainyu's gaze attack.
I don't think this playerbase is the best to have any form of complexity around |D
And if people feel insulted for this, I'm sorry but start proving me wrong, then I wouldn't be saying it.