I tried switching to this editor permanently from vim. The selection based movements are asurdly good, I could kill for something like this in vim. However I couldn't stick with it for two reasons. The first is when I went to figure out how to customise it, I found that the customisation is essentially vim like but weaker. Everything is hardcoded like it would be in vim but with less options. If you're used to Emacs…
Glad to hear you liked the interaction model, may I ask you what kind of options were missing for you ?
The extension model is, I agree, unorthodox, but I think it does work reasonably well (the file you linked manage to provide asynchrounous clang completion and syntastic like diagnostics using it), and keeps things simple. Note that we try really hard only to depends on POSIX tools, so the scripts are actually targeting POSIX shell.
The rational behind this extension model can be found there: https://github.com/mawww/kakoune/blob/master/doc/design.asci...
And a more in depth explanation on how to use it is there: https://github.com/mawww/kakoune/blob/master/doc/interfacing...