>>53132534SI gets hate because inserting yourself as the main character can lead to a power fantasy, cringe, or often a blend of both. Given that this is fanfiction and the majority of writers are far from pros, a lot of SI authors tend to try and boost the likeability of their protagonist (themselves) by making them extra special: mastery of aura that makes a pack of Lucario jealous, the attractiveness of Adonis, born an unstoppable battle genius, etc.
In the worst cases you have authors completely butcher existing characters for the sake of elevating their SI, either in error or sometimes deliberate if they happen to despise the character in question. Everything, from existing characters to the plot itself, is thrown on the sacrificial pyre for the benefit of Tyson, 25 year-old turned ten-year old master psychic, aura-user, and lady pleaser.
Now OC protags can end up the same of course and sometimes readers like a bit of powerwank (I find Borne of Caution to still be a decent read despite it for example), but a lot of SI's go overboard with it to an intolerable degree. At this point there's so many SI's that fuck it up and add shit like random magical katanas to make the protag 'cool', seeing 'self-insert' is just a red flag for a lot of readers.
