By that same logic, every story that Suethors write shouldn't need any constructive criticism or any alteration in the way they handle the characters or the way that they craft the story, because they're writing what they want to write. Perhaps the pacing, or the spelling, or the grammar could be improved, but if the author wanted to have Gimli, Mario, and Rick Deckard all fuse into a Voltron-hybrid that falls hopelessly in love with Rukia Kuchiki, by that logic no one would be allowed to say "Hey, um, that doesn't make a lot of sense." If people were writing something that they didn't want to write, they wouldn't put it online. Sometimes, though, or debatably even most of the time, people need to change aspects of their story to make it better, even if it means compromising their Soul Reaper-on-horrific abomination makeout sessions.
I'm not saying that this crossover would necessarily be badfic. I was just using a hyperbolic extension. I hardly know anything about the story plans other than they involve a galactic empire in some capacity, and it would be possible, albeit enormously difficult, to write goodfic using nine canons crossed over. It's just that there needs to be a lot of planning for this sort of thing. If a story crosses over into the Star Wars universe, there's no need to have the Serenity swoop in at the last minute to save the day when the Millennium Falcon could do just as well. If it's taking place in the Harry Potter universe, there's no need to cross over with the Fullmetal Alchemist universe just to have Edward Elric show up as Defense Against the Dark Arts teacher, unless there are reasons for it and it's a significant plot element in the story.
It's not only the risk of redundancy, but there's also a similar risk that if a canon was added only to include a certain element, it will create plot holes if related elements are not carried along with it. If GLaDOS exists in Middle-Earth, she had to come from somewhere, so omeone would have had to build her, so there would have to be knowledge of robotics, and if she was replacing Smaug in the events of the story there would need to be some sort of explanation for how she was able to clear out the entire kingdom of Erebor, etcetera etcetera, because if you don't, there are things happening for no reason, and that's not very conducive to good storytelling.
Those are some of the big ones, but there's also a chance that one canon is going to eclipse all of the others it's attached to. That's less of a risk, because an author would most likely not cross over between multiple worlds if they didn't have a plan for what to do with each one, but if a crossover included Little House on the Prairie and Puella Magi Madoka Magica, I'd be willing to bet that it would spend a lot more time in fighting witches than in chopping firewood. Again, not as big a risk, especially since one canon eclipsing the other may have been part of the story's point. Maybe the big idea between crossing those two over was to get Laura Ingalls's perspective on being turned into a Magical Girl. It's an odd premise, but there's probably a good story in there if the idea was given to the right person at the right time.
A crossover should tell its story using its resources in the most effective way possible, or it's going to leave all kinds of loose ends. That's still not to say that it can't be done on a large scale, but the technique might be different, and again, there would need to be plenty of planning, as well as a premise that accepts the multiple worlds.
If it's a crossover like the PPC, for instance, and the reason why the wizard and the ten-foot robot with a warhammer are in the same room together is because they were dropped into the same place by dimensional portal and they became good friends, that's fine, because the rules and standards of their respective universes only extend to the characters, and their backstories don't necessarily need to make sense when laid next to each other, because the question of "Wait, how are there wizarding schools in the post-apocalyptic future the robot comes from in which all magicians were killed by nanobots?" is precluded by "the wizard isn't even from the same universe as the robot, and uses a different magic system that would not leave him the target of the nanobots even if he went there for a visit". Or, if it's exclusively for humor, it might not need to make as much sense, and GLaDOS can hand-wave the lack of Dwarves in Erebor by saying that she flushed them out with deadly neurotoxin, with that being understood as a joke based off of the Portal reference without the requisite necessity of knowing where she got the neurotoxin in the first place. It would still need to have some sort of meaning in-universe, though, or it would be the biggest of lipped alligator moments and wouldn't come across as all that funny.
So, to make a point that I was intending to make at some point along the line there, never got around to making, and decided to put at the end, I'm not trying to tell Seychelles what to write, as you seemed to be implying. I don't know enough about this proposed story to do that even if I wanted to. I was telling him or her that writing a story with nine fandoms involved houses all sorts of storytelling risks, especially if my previous suspicion is correct and the story was initially written for two, and that in order to tell a story with that many worlds co-existing, there would most likely need to be some concessions, to make whatever the plan is work best. That might mean changing the way that the canon worlds interact, it might mean dropping some characters to keep them from solving problems instantly, it might even mean dropping some of the fandoms being crossed over.
If Seychelles decides to write the story without taking that advice into account, I'm not going to impose my ideas on him or her. I don't think a story told in that way is as viable as it would be with fewer fandoms, but that doesn't means everyone does. Some writers might like to challenge themselves by writing a working story in as many worlds as possible, or maybe there's some really, really good idea that only works if someone is capable of interacting with elements and features of several different worlds simultaneously, and I just don't know about it and consequently haven't taken it into calculation. But, rather than adopting your philosophy and saying that all ideas are good on their own merits if the author enjoys those ideas, I decided to give a little advice that I felt would be at least worthy of consideration in the majority of situations, because my beliefs tend more toward the front that while almost any story can be told well, the execution will probably be better in a smaller but more interconnected story than in a larger sprawling one. Another belief: people writing what they enjoy is great, but writing something that both they and others enjoy and can follow is better.
Speaking of that, crossing over nine fandoms would probably also drive away potential readers, since the majority of the audience might know a few of the fandoms but not the rest and would decline reading the story because they feel that they would have no idea what was going on. It's not the biggest of deals, especially since most fanfiction isn't written primarily for traffic, but maybe I should put it up there somewhere. (looks up at five paragraphs of text) Nah.