That statement is extremely subjective, but it's mainly just bad blood with how the story was handled (mostly players were upset with how the Ancients/Ascians and Garleans were treated - there was much disappointment Garlemald was only visited after it was already destroyed by it's own hand,
and with the Elpis time loop complicating things and Venat's reasons for the Sundering being actually morally murky, retconning everything that had come before, and that it is impossible not to raise an eyebrow at the rather ludicious concept that everything that is wrong with the universe is simply because a space probe got depressed.
).
There was also a lot of upset players more annoyed with "lack of content" (read: hard end-game content) in the post-launch patch period compared with earlier expansions, and changes they didn't agree with to certain classes and Jobs (SMN, healers etc). My personal take is I still thoroughly enjoyed it even despite a couple of minor gripes (again connected to some very minor odd retcons in the storyline), but I still consider it a fitting finale to the '1.0-begun' storyline.
Everyone's own opinions are unique though so don't listen to what others say and make your own judgement.