The problem was also the way they fit all the characters stories together, look at the first two Batman movies.
They introduced Ras, Bruce, Alfred, the lawyer girlfriend whose name escapes me, Scarecrow, Gordon, and the Mob Boss, all one the first movie.
They had Bruce, Alfred, the girlfriend, Gordon, Dent, and Joker, in the second.
Lots of characters, all worked together nicely.
Originally Raimi wanted Vulture instead of Venom (or so the rumor says) and they would have escaped together and Sandman would have been the unwilling muscle to the Vulture's brain, so the story would have worked better (somewhat).
So its possible to do that many characters and have it work, if the script and the character's stories fit together better than Spider-man 3 had them fit. In my opinion.