Cosmo
Does Not Know Kung-fu
In my experience, one of two things usually happens:
1. The author's self-insert/pet character becomes their mouth piece for their own personal beliefs to the point it becomes vary jarring [Even if you have similar views] and the world will noticeably bend, even when it shouldn't, to match their narrative. This makes the story telling feel extremely clunky, preachy and tends to make you not like the character on principle. [Chris Avellone does this a lot with characters like Kreia (Kotor 2) and Ulysses (Fallout New Vegas)]
2. The character tends to have noticeably more plot armor and tend to be a bit Mary Sueish.
Often both happen together.
1. The author's self-insert/pet character becomes their mouth piece for their own personal beliefs to the point it becomes vary jarring [Even if you have similar views] and the world will noticeably bend, even when it shouldn't, to match their narrative. This makes the story telling feel extremely clunky, preachy and tends to make you not like the character on principle. [Chris Avellone does this a lot with characters like Kreia (Kotor 2) and Ulysses (Fallout New Vegas)]
2. The character tends to have noticeably more plot armor and tend to be a bit Mary Sueish.
Often both happen together.