I think the Customizer is pretty confusing. It's location is somewhat hidden, since it's a submenu item in Appearance > Customizer. Then when you click that you are greeted by a completely different environment, as if you are outside the Wordpress admin panel. And then you find the different customise options, like setting the static front page, setting menu's, etc, all options which can already/also be found elsewhere in the admin panel. Having worked with Wordpress for ten years, have never used the Customizer myself. And I know my clients (non-tech people) cannot use it because it's too complex. Neither do I want them to use it, since I think the design of the website is something the web designer should be responsible for. The user should be concerned with the content of the site.

The problem with the Wordpress admin UI is that it is build for completely different groups of people using it.
- experienced web developers tech savvy enough to deal with complex things like decing on the kinds of permalinks etc,
- non-tech users who are just content editors and don't need to do more than create/edit posts and pages and working with uploaded media
- semi-tech savvy users who want to build their own weblog/website, doing both "development" in the sense of customising themes and plugins but without PHP knowledge, as well as creating and editing the content of the website.

Not sure what the best solution is. Maybe it should be possible to have three different kinds of admin panels, targeted at those three different groups of people. A bit like you have now with different user roles viewing more/less. But then specifically build with those three groups of users in mind.