Web Audio API: is granular synthesis alone enough to do high quality pitch shifting in JavaScript?

I found this project on GitHub: https://github.com/urtzurd/html-audio

At the demo, their app, while their pitch shifting is functional, the audio quality is horrendous.

What are they doing wrong?

No external libs or frameworks please (this is a constraint put into place by my superior).

Edit: before anyone who doesn't know about this answers, both detune() and playbackRate modify both the pitch and playback speed. One exception is using preservePitch() with playbackRate, but I'm looking to modify pitch and not playback speed.