the generated webpack config is an array, not an object. That’s why the merge function returns an object with the index 1 (which is the unknown property that webpack complains about). You should probably just fix it by wrapping the object that you put in the merge function into an array.
Just now seeing your post. In case you still looking for an answer to the sass question, here’s a snippet of what my webpack.config.js looks like wrt sass support: