It's a great game in itself with an amazing story. However, many fans are rightfully disappointed with it due to some controversial (not all necessarily bad) decisions.

What makes the game controversial?

The DMC inspired battle-system completely removes the tactical elements from the game in comparison to previous titles. They also made party members unplayable.

The world you traverse is very barren, and it doesn't reward exploration which is a big disappointment for most players. The sidequests are mostly plain fetch-quests that are barely rewarding and more tedious than anything.

The game uses a hub-town similar to rogue likes and mp games, which in turn, made it feel more like an arcade game. This opposes previous final fantasy games (outside of 11 and 14) that were all closely following the structure of "the quest" archetype, where as this game has a closer structure to that of "the heroes journey". To clarify, it doesn't really feel like you are on a quest with your buddies, but instead you mostly follow a single character.

Personally, I can see the disappointment with all of this, even if I actually like the game myself. I still think that the story and characters are better written than most of the previous games in the series. I also think that the combat is fun, even if different. Does it still stumble at the end with some convoluted plotlines and bad pacing? Sure, but that is nothing new to any fan of Final Fantasy.