I have heard so many horror stories about editors and experienced a few myself. If an author is uncertain about an editor, most editors will do one sample chapter or 10 pages (whichever comes first) for free to give the author an idea of their style. Edits can be very intense and I would encourage authors to listen if they keep hearing the same things about their work from different sources. That said, they also don’t need an editor who is so brutal they never want to pick up a pen again.
Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.