"Ergonomics (i.e. edge of the screen is fast to reach with a mouse, color contrast, size of fonts, ...)"
It's more basic than that - the premise of UI deals first core usability, the layout of information of components in a manner that communicates information and allows for basic interactivity, and then to make it all coherent across the app.
That alone can get pretty hard, there's all sorts of decisions that need to be made. Consider that it's as much about what you don't see as those elements you do.
Then you can get into the more granular aspects such as those covered in this article that I identified as 'picky' (for example, the specific contrast ratios among various elements - this is far beyond what 99% of UI designers have the luxury of addressing) and then of course broadening the scope of accessibility, corner functions, cross platform consistency.
Even the basics of UI can be pretty hard, let alone the aesthetic elements, and very specific things.
And of course, all of it 'cross cuts' with the aesthetic aspects, which is why the disciplines of UI and classical design are so tightly interwoven.
Design and UX is very poorly understood, and it's difficult to professionalize or measure given the abstract nature of the subject. As Engineers, we love to measure things, and much of great design defies the kinds of classical measurements we are used to. As the OP indicated, it's also a field wherein it's hard to be self aware i.e. to 'know what you don't know', and so 'everyone has an opinion'.
A great UI is almost like a great API - it's hard to measure and articulate, but generally we know it when we use it.
