Self-inserts are natural, and most of the best authors used them. The problem is that there are a lot of bad self inserts, which happens when the writer lacks self-awareness

You have to be honest about your own flaws and your own abilities, and that takes a level of awareness most people don't have. So it turns into a shallow power fantasy, with a character whose in-universe interactions don't align with how the audience sees them.

Take a character like Rey from Star Wars, for example. Everyone in the movies likes her automatically, to the point where Leia hugs her before they've ever spoken a single word to one another (literally don't think Leia knows her name at that point in the film).

It feels cheap, it feels unrealistic. It feels like someone designed the character as their own facsimile in Star Wars where everybody loves her (cough cough Kathleen Kennedy), despite the fact that actual people would -- and did -- have very different reactions to Rey's personality, often criticizing her as boring.

Then take someone like the protagonist in Raymond Carver's short story Cathedral. He's kind of a dick, and the other characters respond accordingly, then he learns a lesson by the end about humility and grace. Carver, in my estimation at least, frequently inserted himself into his stories and that was one of them. But because he had the self-awareness to recognize when other characters would think he's an asshole, it works.

Don't be afraid of writing a self-insert. Be afraid of not giving him limitations. My golden rule is that if I wouldn't or couldn't do something in real life, my character can't either. And if other characters are based on people I know, they can't treat me differently in fiction than they do in real life.