Like the others have been saying, the problem with Disappointment is that it's challenging to make up for the lost aid of the faerie, and having to re summon her takes time and MP (Something that SCHs have a harder time with in Stormblood).

I have an interesting suggestion that might turn Disappointment into a better skill. Rather than the way it currently works, I'd like to see it swap your faerie and provide a buff to you and to her.