The new customizer should be completely optional or a filter to enable older full width version without preview should be supplied.

Just add some 20+ images as random header image with the new customizer. Or 6+ menus with 50 entries each for different site areas, or 10+ widgets in 20+ sidebars.

Impossible to handle with the new customizer for people without deep WordPress knowledge and understanding. But in real life these are the people who should be able to add content to a WordPress site.

If you use WordPress as CMS instead of a "look-at-my-latest-video" Blog, the new customizer is a perfect example of very very bad UX.

Things like the new customiozer kill the simple and straight-forward usability which WordPress is/was famous for.