Don't let anyone else tell you different. It's really hard. You've got to walk that fine line, that razor's edge, between the writer's vision, and your own. You can see the writer's blindspots, but they can sometimes see yours.
Writers have just done something remarkable, and it is the rare writer who is fully open to having that word-baby torn apart by the jackals of judgment and style. But without that gnashing and bleeding, the book is likely to have some problems. Hopefully the writer acknowledges it can be made better, and hopefully the editor will not scar the writer's unique feel with their own personal style. It's a hard, hard balance to strike.
I am presently engaged in the process of editing a work of nonfiction. Structure may be what determines success. I may have to write or encourage the writing of an acrostic poem for greater coherence. How often can one say that?