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 amount of time I spend in a specificity war between theme styles and wp core's ever-changing sheets is far too much!
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... :(
