It’s probably a given that the majority of authors can only go so far editing their own work. It’s partly a question of being too close to the page and the story but even if you put the MS away in a darkened room for a period and return to it you may still find it difficult to pick up every misspelling; every typo and grammatical error.
One reason might be that since you know every wrinkle, nuance and minor event in the story your eyes and your brain tend to skip over the words. You know what’s coming and you can anticipate events.
A good editor picks up everything, line by line, mistake by mistake.
I have experimented with using beta readers that offer their services via sites such as Goodreads.
My experience so far is mixed. One beta reader gave me as tough an edit as a professional editor (maybe she was one). No names no pack drill but I was impressed that she’d even gone so far as to check on Google Earth for locations mentioned in the MS and whether a good claret is ever served by the glass.
She was superb. I doubt if she missed anything.
On the other hand, one beta reader just gave me a single page email opinion picking up one or two minor points.
I am expecting a couple of other beta reads in the near future. I will then combine them with the first and go through the book again. I expect there will be a lot of duplication but every tiny error that is picked up is like gold dust to the author.
So, the jury out as to whether a beta reader can do as good a job as a paid for editor and I suspect some beta readers are professionals offering services by the back door.
I’ll keep you posted.