Stormblood gets a lot of bad rep for not being as good as Heavensward, which is the expansion that really took FFXIV and ascended it into the realm of truly epic MMOs. There are some pretty glaring flaws, like turning a fun ally in Yda into a flat and somewhat dull personality, the main villains being little more than Generic Bad Guy tropes (though this changes later for nearly all of them, yes, including HIM), and a rather predictable storyline for most of the expansion. But it's better than ARR's storyline where you're still an unknown adventurer and are stuck with fetch quests. If Stormblood released before Heavensward, it'd be remembered a lot fonder for being a flawed yet fun story, but it came on the coattails of the equivalent of "American Idiot"; how can you possibly follow up on a success like that?

Good news: Stormblood's post-game MSQ is fantastic and contains one of the best fights in the game, so be prepared for that.