This Question comes on the heel of wasted money and confusion. I am committed to writing the best books I can + to getting published. I love words. I love the world of words. I read authors whose works inspire + teach me. I have solid critique + beta partners. On occasion, I'll take a class online or otherwise. On those occasions, I'll look into the background of the instructors + editors to ensure there aren't any crackpots. Here's the question + the rub. Twice I have worked with paid editors and twice I have gotten either bum advice such as: you don't need to tell the ending in synopsis; or a critique that would have changed the body of my work so dramatically as to be a Dementor's Kiss. Thinking an editor should be seeing the landscape, I worked with (some) of their recommendations only to find that, yes, the soul truly had been sucked out of the story on their (paid) advice.
Does this happen to author's with whom you've worked? Does this happen frequently or is it only "paid" editors? (Is there a difference) because I'm getting jaundiced on them as a whole. (PS. I've since written the soul back into my work.)
It doesn't happen with editors at publishing houses because if they want to suck the soul out of a manuscript, we have a conversation that involves changing editors or moving the book to a new publisher. My job is to find an editor who actually likes the book, not one who wants to change it completely.
Outside/paid/independent editors are a whole different kettle of fishies. I've had terrible luck with most, and great success with a very few.
How to find the latter and avoid the former? READ the books they've edited.
Also, have a clear idea of what you want the editor to do. Do you need the plot strengthened, the dialogue improved? The pacing quickened?
Often an editor can make suggestions about how to do those kinds of things without going through the entire manuscript page by page.
If you're looking for someone to read for plot holes or narrative arc, then you do need someone who will read the entire manuscript.
Good editors are not thick on the ground. Finding a good one is not easy. The REALLY good ones are booked up so far in advance, even their pals can't get a project on their desk (I'm looking at you Kristen Weber!)
I wish I had more to offer on this topic but it's an ongoing problem here too.