If by tested you mean, do the styles look the same as CMB2/master? Then the answer is yes.
I tried to keep the CSS identical to what you had. However, I did make some minor color tweaks to keep things consistent (was there really a need for 2 or 3 different "light yellows")?
And it looks like it does keep GIT submodules current, not Node modules. https://github.com/jaubourg/grunt-update-submodules
Yah, that's pretty much what I mean. BUT CMB can be used in the post metaboxes, as a settings page, as user fields, and not to mention the front-end. so If we're going to make big changes to the css, we need to at least look at CMB in all those contexts, as well as viewing each field type. Does that make sense?