I have used Kakoune. I tried it out pretty extensively for a few weeks. I could not get comfortable with it. I've used vi and vim for two decades now. Kakoune seems like it would appeal to people who have moderate exposure to vi/vim but not mastery.

From the mastery standpoint, Kakoune seems like a big step down from the vi model. Too many of its commands/selections seemed to give priority to the edge case over the central one. Most of the text editing I've ever done with vi is of the garden-variety "navigate to this line and insert/delete/change one thing". Kakoune makes this more difficult by creating multiple cursors/selections as a side effect of its movement/navigation tools. It's like Kakoune is trying to tell me "no, you want to edit stuff all over the place!"

I also think the promise of Kakoune, that you can use selections to see what will happen before you type the command to actually make the change, is violated when there are selections outside of the visible portion of the file. Furthermore, with vi/vim I tend to type commands so quickly that the visual nature of Kakoune would only slow me down, and so at that point the advantage of selections is greatly diminished.