In practice, most designers know what they are doing as well as you know your job

I would qualify that as most graphic designers know what they’re doing, when they’re working in graphic design media: logos, magazines, billboards, and other print media.

Unfortunately, our industry (software) has shifted towards hiring graphic designers to do user interface design, a task which they’re often grossly unqualified for. This is how we get interfaces which are attractive but frustrating and difficult to use, both for beginner/non-technical users and for expert/power users, for different reasons. Beginners need intuitive interfaces with high discoverability for the most important operations and power users need efficiency and clean access to automation tools for the most common tasks.

Graphic designers often sacrifice power user features entirely in favour of minimalism. They also sacrifice intuitiveness in favour of their own artistic vision. In both cases they frequently sacrifice information density in favour of minimalism, making it hard for users to quickly locate the information they need. This is especially egregious on mobile-first designs where the accuracy of mouse pointers and the information capacity of large displays is ignored.