Paperpile is fantastic and a joy to use. Buttons and menus are mostly where you expect to find them and relevant info pops up where you need it, and it works consistently and quickly compared to other browser based bibliographic software.
But there is one big problem visually, and that is the big, distracting RED button in the sidebar to “update citations and bibliography”. Why red? Red is a warning color that signifies that an error has occurred that must be fixed. How about green or blue? I keep the sidebar closed solely because the red is so distracting. I know you have bigger fish to fry, but this one would be really quick to change, should you agree with my reasoning.