In our book publishing company, Expert Publishing, Inc., I’ve had the opportunity to work with several editors. Sometimes I’ve had to re-do their edits because they changed the author’s intent. Obviously, I don’t rehire those editors.
Every author (including me) needs an editor. Authors know their topics so well that they don’t know when they’re not being clear to the reader. Authors also may not know the various editorial manuals as well as an editor does (for example, periodicals don’t use the serial comma–the comma before the conjunction in a series–but books do).
One editor we hired was very well versed in the Chicago Manual of Style, the book publishing industry standard, but he changed the author’s voice in the manuscript so the text read the way the editor would have written it, not the way the author wrote it. Of course, you expect your editor to improve your work, and you should trust your editor completely, but the final decision to make a change should be yours, as the author whose name is on the work.
Another editor we hired worked on a book that contained several scriptures. Chicago says to include the translation (KJV for King James Version, for example) next to the scripture reference. The author did that and the editor changed Version to Volume. Clearly, this was not an editor familiar with scripture. Had we not checked her work, it would have been a disaster for both the author and us, as publisher.
By all means, hire an editor when you think you’re in the final draft of your manuscript. BUT never allow the editor to change your intent. The editor’s job is to improve your work. You are the author and your name is on your book or article, so it should reflect your words, not the editor’s.
On the other hand, hire an editor you trust to challenge you to be as clear to the reader as possible. You won’t go wrong.