No the main difference is that vis is written from scratch while neovim inherited an old and hard to maintain code base.
This allows vis to experiment with various ideas:
- native multiple cursors/selection support
- structural regular expression support
- different core data structure resulting in efficient large file support
- LPeg based syntax highlighting
- Lua as an in process extension language
- client/server design (not yet implemented)
without having to care about backward compatibility and legacy systems. Of course this has also some drawbacks for example there is no existing plugin ecosystem etc.
I'm not really familiar with the vim code base. From the little I've seen I'm astonished that people are actually willing to voluntarily work on it.
Having said that, googling revealed a bug report[1] where one of the main neovim developers states he would rather avoid "messing with a very obscure part of the C code." So yes, it seems to be difficult to implement into the existing code base.
There exists a plugin which adds some features, but because it is not supported by the editor core (hence the above emphasis on "native") it isn't properly integrated with other plugins etc.
