This is still a fairly significant issue in other scenarios. Any part of the screen that is rendered via the router will render before CSS is loaded. For example, the main dashboard's header, body content, etc will render immediately since it is not actually being rendered via the Extension.Renderer component. The only React components that will render after the CSS are loaded are those that are contributed to an Extension.Renderer.
We had some discussions Keith Zantow and Nicolae Pascu where we concluded that ultimately it may easier (and more performant) if all the CSS was loaded upfront in a single file. This would block any initial page rendering until CSS has finished loading and avoid any of these timing issues. This would require some additional work to concatenate the CSS (or Less) from multiple plugins and place the file in a known location that can be loaded via link rel=stylesheet tag in blueocean-web's index.jelly.