I think it’s great for authors to multi-task. Sure, write, edit, design, and create your own book covers. It’s lots of fun for sure. But, should an author edit their own work?
I mean, and don’t get me wrong, a writer should read over and edit to the best of their ability, but don’t publish it until an unbiased eye looks it over. Make sure those who read it love to read your genre. Don’t give a horror story to someone who reads romance and hates the sight of blood.
According to Stephen King you should give your manuscript to at least ten people to read and then listen to them with an open mind. Take to heart what they tell you.
An author will look over some critical points. They don’t see mistakes clearly and will overlook key points in the story, spelling, and word usage.
A trained independent eye will see them.
If you wear too many hats you will look like a genius unless one is crocked.