No, it mostly suffers from the problem that the people who write it are trying to create Art, and that never ends well.

Or, to answer the question you're actually asking, I'm arguing probabilities, not absolutes. Good fanfic exists - hell, we're on a thread to debate a fic sufficiently good that it caused me to read the original Potter books(seriously, the number of references in those first 30 chapters I missed the first time is kind of staggering). But it is not the majority.