AU Controversies
Many fan fiction readers and authors dislike AUs because some AU writers disregard the canon of official work by or approved of by the author. However, some authors specifically write AU fiction instead of textbook fan fiction in order to explore issues which were not fully addressed in canon, and may present a meticulously researched canon universe which then veers away from actual events in ways which logically proceed from the changes the AU author introduced.
Another type of AU that authors and readers have problems with involves the storyline of a well-known movie or video game, played out by original characters or by characters from another series. An example is "A Different Sort of Adventure" by ImLucky, in which original characters work through two of the Pokémon game adventures. Finally, some people have theorized that because AU authors are already radically changing the story as it is, they find it far easier to add a Mary Sue, or, in other terms, a wish-fulfillment character.
A common mistake made by inexperienced fan fiction writers is to believe that writing an AU fan fiction means that the writer can acceptably and drastically alter the personalities of major characters; in fact, the point of AU fan fiction is that the characters' personalities remain as much the same as possible, and the only changes are those which would rationally be caused by the differences from canon.
Read more about this topic: Alternative Universe (fan Fiction)