Any harnomy process will have some mechanical tones - even the most expensive device like a TC Electronics (I know I have one in additional to a Digitech).
I find that if I have the "real" lead vocal and add one "real"
harmony (whichever harmony is the most important) then the machine can fill in the rest with a degree of realism.
However, if you hope to depend on the machine to be a magic backup group, you'll be less than satisfied.
By the way, having owned both, I'm not convinced the TC is that much better than the Digitech as a "realistic" harmonizer - however the mic pre, reverb & compression do add to the value of the TC.