If you want to make fic, it should have followed the rule of the world that believed by reader. But people forgot that author are the one who build the protagonist and the other characters.
Some author hate it when the depth character they made get tampered with according to fanbase depraved desires for the sake of their own shitty fanfic.
And crossover ?
It's basically a child playing barbie doll while the other child bringing the superman, batman, and spiderman doll while the first child getting those Gundam figure from his brother's locked Display cabinet with 1:1000 US Navy Ship model bottle and the other child also getting the Rem sexy figurine and Hatsune Miku figurine too from hat same cabinet.
After that, All the toys, mecha figures, ship models, and sexy figurines were scattered on the floor and most of the items were permanently damaged and had broken parts so everything looked so messy.
Even official crossover don't usually making good result. It's pretty much a chaotic balancing act.
Official crossover always made it for each character to kept everything like canon with accurate portrayal. But it prevent any changes to any kind of personality development from any sides character, even if theoretically they should've been affected greatly and deeply. Especially if their meeting could possibly cause life changing trauma that can change person personality as a whole, but the author just ignore it and keep the character locked "in-character" until the story is over with open ending.
While crossover always show how much possibility those character can have, but it depends too much on the author's shitty imagination, which is usually not that bright. Character will always go out of character since author didn't know any deep detail and possible spoiler hidden from reader. Their interaction is also very bad, it's always exaggerated and out of control. And there's also author's wishfulfillment and their ideals, complete with their own belief and political agenda, and they stole official characters to portray those shitty belief and write them act for those messages, even if said character never exposed to such ideas in their lifetime in official story.
