Gutenberg brings React and a block architecture. That blurs the lines between "chrome" and end-user styles. This is more at the CSS and React level. The amount of !important CSS rules going into core is kind of terrifying.
The block library stylesheet is awful, and there is also no interface to interact with it directly. Instead we get to edit a json file which gives us access to a few dozen properties and deal with that.
Not to mention the incomplete javascript and PHP interfaces which seem to be developed by two battling teams... :(