One counter example is “Harry Potter and the methods of rationality”. Possibly one of the best books I’ve ever read.
Worm is an awesome tale that have mostly destroyed other stories about super heroes for me.
I really admire the work ethic of the author, he has been delivering high-quality chapters every week since 2011, and you can just read it online for free.
On the other hand it shows a lack of editing. There are whole chapters that don't move the story forward (like a third of the book is dedicated to student battles that just seem like a filler before the author decided how to end the story). I feel like those would all get significantly shortened if this would be a published book with an editor.
Anyway, I like reading fanfiction, but I think it stands on its own and it's hard to compare it with conventional published literature.
I've always been frustrated in Harry Potter that there's not more writing about the lessons for example. There's quite a bit about it in the first book, then it's more and more scarce.
The books are incredibly entertaining and actually spawned a number of short stories and novels by other self-publishing authors set in the same universe.