I have done it, PitchFactor, H9, and now the H90. Definitely (and logically), a significant increase in quality over the units. The H90 versions adds a Quantize parameter, that I like. It’s no MeloDyne or TC Helicon for vocals, but I can see where it could cover for a more synthesized output. I’d say some kind of post-processing certainly helps out vocal input, to tie it all together. And you might want some kind of LP filter, once you get past +1 octave.
In QuadraVox, you can adjust the intervals for all Voices: A, B, C, and D, constrained to the selected Key & Scale. Voices A & C are panned left; Voices B & D are panned right. Diatonic pans A & B. The difference is more control over the independent Delay parameters, plus a cool Feedback routing. QuadraVox treats delays as a group, with gradations in spacing between delays. Delay D setting your longest delay time.
https://cdn.eventideaudio.com/manuals/h90/1.3.0/content/algorithms/pitch.html#quadravox
https://cdn.eventideaudio.com/manuals/h90/1.3.0/content/algorithms/pitch.html#diatonic