Line Editing a book and re-reading it until my eyes bleed making sure the grammar and continuity is just right or audio editing a recorded song and listening to slightly different version over and over again until my ears bleed to put together the right mix.
I've done both professionally and I still don't know which I prefer. The parts of this book I did with BookNova as a test are far worse than what BookCrafter added after because BookNova's was over 30,000 words too short and did some weird stuff, but the story was worth saving. I think. We'll see.