I didn't try it. But I surfed the website and watched the animations.

I am using stock Neovim on the console. For completion I switched to nvim-cmp https://github.com/hrsh7th/nvim-cmp. Onivim also provides (based on the screencasts) completion. Do they leverage their own coded completion engine or are they using nvim-plugins and only render the UI over this completion?

My point is there will always be an impedance mismatch between functionality provided by plugins compared to what a UI provider thinks has to be coded in their respective implementation.