The stand alone units are excellent quality voice recognitions & multifeatured for many types of harmonies.....but its another thing to carry & midi up in your rig rack......

For onboard the Roland G70, Discover 5, Ketron SD1+ are way above the rest.

As there isn't a magic pill for vocal harmony believe me there is a whole other ART FORM besides playing and singing lead to using and generating Vocal Harmonies correctly and believably when performing.....I was taught by the best Uncle Dave .....listen to this & you'll know what I mean.
http://www.boydsongs.com/A%20Nightingale%20Sang.html