I've found with both AutoTune and Celemony that the greater the correction, the more notice the artifacts. I prefer Celemony for editing as I find it a bit more useful (and the polyphony editing for chords is helpful... although it can get a bit ...spacey
a few times the vocalist was so far off, either pitch control just "moved the problems"... so to speak, so i had someone else sing the song (in pitch and key) and had the vocalist practice to that, even once using it as a 'sing-along" scratch track... this got the vocals closer so the pitch correction wasn't as noticeable...
someone once told me, the best approach is "practice, practice, practice"... or change the key of the tune to something you can sing...