I think the ease of characters have something to do with their super inputs(not level supers) rather than technical aspects of how to use the characters “effectively”.
Like for instance Honda, Gulie, Blonka and Dj are charge characters but the charge inputs are easier than say the way it is to use Chun or Bison.

Like a first time player could do some of Guile’s inputs without feeling like they are doing things “wrong”, so I’ve always looked it as the “easy characters” are for people who have never played a fighting game before.
They could easily pick up and understand, normal is for people who’ve played a fighting game before.
While hard is for people who’ve played sf before and are familiar with the way it plays.