I haven't had tons of experiences with different keyboard harmonizers.
That said, I have used Digitech and TC Helicon harmonizers with guitar...prefer TC Helicon.
My wife and I found that we needed different settings for her when she used both the S950 and Tyros 4 harmonizers...different than mine I should say.
Here, my wife is using the TC harmonizer system of the Tyros 4...starts at about 42 seconds into the clip.
https://www.youtube.com/watch?v=ubf-OJE_Xd4The mike on the camera was so so in the clip.
Harmonizers do take a bit of tweaking regardless...at least the ones I've used.
Also, no harmonizer sounds as good, to my ears, as real vocal harmonies.