It's pretty understandable with Hades' situation in KH2, given what Dandelion said. Still...for some reason, it seems underwhelming. But while we're on the topic of Disney villains, another example of lackluster, is the oh so powerful 'Mistress of All Evil' herself.
Maleficent was hyped up to be the big bad in KH1, but when you ACTUALLY fight her...she's BARELY a challenge. Her Dragon form, there's no debate that she's a challenge, but her regular form...she has Magic...and the best thing she can do is summon easily avoidable meteors, summon heartless, and summon electricity? I mean...if Final Fantasy was in the mix in this game, it would've been a nice idea to let her cast some powerful Final Fantasy spells...perhaps even DQ spells (But I'm getting ahead of myself there.)
I think it might have something to do with gameplay and story segregation. A lot of the Disney villains are actually quite powerful in canon but are trivially defeated in gameplay. Maleficent should had a been a bit more difficult than she was in KH1 but instead she is just a complete joke of a boss. Even without gameplay, the Disney villains should had been more of a threat to Sora and his friends.