Gemstone's books didn't sell badly; nothing like as well as the best-selling superhero titles, of course, but still pretty okay. If Boom's "classic" titles didn't do quite as well as hoped (not sure about this), it's probably because, as you note, they sort of dulled the momentum with, well, things like Ultraheroes.
Let us note, however, that it wasn't JUST a matter of them publishing "genre" material. Remember their two Uncle Scrooge serials, "The Hunt for Old Number One" and "Around the World in Eighty Bucks?" "Classic" characters, sure, but the stories were just which makes me think there was something fundamentally wrong with Boom's thinking even beyond the compulsion to, I guess, try to appeal to non-Disney-fans.
Eh, I liked the first few chapters of Ultraheroes. The art style was loose and fresh, and I liked the novelty of seeing the Disney characters in a superhero setting. It wasn't interesting enough for me to keep buying the Ultraheroes title, but it wasn't a total travesty.
I agree that the footnotes were supposed to be a parody of the continuity-heavy footnote-happy Marvel comics of the 1980s.
Ultraheroes ran for nine issues and featured all the superheroes of the Disney comics universe in an epic clash against the super villains to determine the faith of the world. Those Italians sure know how to make exciting stories. The series was written by Giorgio Salati and features beautiful artwork by Ettore Gula and Roberta Migheli.