I agree with all of this. I encountered one problem when using luminance (either alone or in combination with color) to encode categorical data though:

Luminance inevitably conveys a certain ordering.

A dark black line will be automatically identified as more as important than a light grey line.

This can be desired (and abused), but often I want to present my categories as neutral as possible. I experimented with balancing lightness with other features (like line width) but not with much success.