in and of itself, nothing.
the problem is most authors/artitsts who self insert do so in one of two ways.>put themselves into the story as the most perfect, wonderful person, refusing to acknowledge any of their real life flaws or issues, and/or failing to give their self insert flaws or issues. >insert themselves into a pre-existing character, often erasing some or all of that character's actually history, personality, and nature, and substituting their own.
self insert fiction works when you remember that your self insert is a character in the story, and thus bound to the story, and required to be a good character. self insert fic fails because most who try it are incapable of recognizing that fact and instead warp everything to fit a flat, uninteresting projection.