Ewwii uses NBCL as the configuration language. But it does not lock you into it for configuration. Through the plugin system, even the core configuration language of ewwii can be replaced, and new widgets can be created. Its highly optimized and is extremely featherweight (4.8 MB).
One added benefit of ewwii is the plugin manager. Users can use it to painlessly manage multiple plugins. The users of eww can also easily switch to ewwii while preserving their yuck configs by using the "yucky-ewwii" plugin.
I soon am going to release the new version which bundles a lot of new features and quality of life. If you want to contribute to ewwii, you're most welcome!