Maybe not common, but this year I built and integrated a custom frontend UI kit into the block editor. Using the components and props in custom blocks was easy. Getting the compiled styles working in the editor, not so easy. Had to get creative with postcss-prefix-selector and it all came together.
Navigation in the site editor is for me still one of the biggest pain points. Not very usable and not very flexible.
»Responsive Navigation Block« by Dave Smith is helpful, but it shouldn‘t stop there.