I've run into a lot of "UI designers" that were just graphic designers that lucked out getting a job in a software company. They design and compose the UI for static screenshots like they would do a 2D graphic composition. Far too often they'd hand off Photoshop PSDs they would expect a developer to turn into an actual UI. They don't follow a HIG document or respond to any pushback with "you have to know when to break the rules" (they do not know when to break the rules). A good UI designer is worth their weight in gold. In my experience most are worth their weight in very low grade playground sand.