It's worth keeping in mind that non-fiction and fiction are very different ballgames. (And, I believe, erotica is fairly different from fiction too.)
With technical books, the potential audience size is much smaller, but it's way easier to reach a larger fraction of them. When you do, they are more willing to pay a decent price for a copy. The average quality is also, I believe, much higher.
Fiction has a huge glut of authors. It's more difficult for readers to find an author whose quality and writing they like. If you need to learn SQL, it's easy to search for "SQL". It's a lot harder to search for a certain prose style or "voice". Also, frankly, there's a lot of really bad self-published fiction to wade through.
Because of this, I think self-publishing is a better fit for non-fiction than fiction right now.
Erotica is VERY different. It may look saturated, but well written short stories with attractive covers and good blurbs can still sell incredibly well.
And, with the advent of Kindle Unlimited (if Amazon's payments hold at the current level per borrow) sales plus borrows is proving to be even more lucrative than sales alone.
It's worth keeping in mind that non-fiction and fiction are very different ballgames. (And, I believe, erotica is fairly different from fiction too.)
With technical books, the potential audience size is much smaller, but it's way easier to reach a larger fraction of them. When you do, they are more willing to pay a decent price for a copy. The average quality is also, I believe, much higher.
Fiction has a huge glut of authors. It's more difficult for readers to find an author whose quality and writing they like. If you need to learn SQL, it's easy to search for "SQL". It's a lot harder to search for a certain prose style or "voice". Also, frankly, there's a lot of really bad self-published fiction to wade through.
Because of this, I think self-publishing is a better fit for non-fiction than fiction right now.