From one of my friends on Twitter. The picture below the link really says it all!
Now, that is not to say that there are not good fanfic writers out there. There most definitely are. However, I have to say I have seen every possible pairing in POTC, for example, up to and including such all time favourites as Mr. Gibbs and his pig and some sort of weird Buffy crossover fic. Ok…We just are not going to go there.
Fanny Fae 
A friend literally just sent that, and sadly it’s so true. For every one crossover fic that works there’s a good two to three hundred that don’t.
Right now I’m reading a crossover between Warhammer40k and Neo Genesis Evangelion that strangely works. I’m thinking it deals with the Lovecraftian background present in both.
Some Buffy cross-overs make sense and were based on essays written in books about the show……. *sheepish*
But I know what you’re getting at. It’s not really the strangeness that gets me usually, it’s the poor writing, actually.
This is why I rarely read fanfic in general anymore, and what I do read is by trusted writers or recs. I love me some crossovers, especially, but most are just ridiculous at best. I once read a Robin of Sherwood/The Professionals (late 70s cop-type show) crossover once…that had no explanation. Yeah.
So true.
There’s good stuff, but there’s a whole lot of horrible crap, too.
I LOLed!