I've never met a UI for which I didn't have to google how to accomplish some things, even macOS'

Completely self explanatory UIs don't exist. In the olden days they required manuals. Now we have online help and google at our fingertips, fortunately.

Unlike jokes, UIs are not an end unto themselves. I'm more than willing to accept a somewhat ugly or cumbersome UI if the underlying tool is as good as Calibre.