Flat UI Elements Attract Less Attention and Cause Uncertainty (2017) (nngroup.com)

The example image included in the synopsis seems a bit strange to me. It's not like they are comparing apples to apples - what they've done is essentially made the control elements blend as much into the page as possible. I don't believe that UI elements have to be of the ghost variant in order to be considered flat. They've taken away the 3D effects but they've also inverted the color scheme. As a result the very noticeable purple button became a white button. This seems to me as an unfair comparison - just removing the 3D effects would still make the then purple button fall under "flat design", no?

EDIT: It seems like in most other examples they didn't do this: https://www.nngroup.com/articles/heatmap-visualizations-sign....

On the other hand however you can argue that just changing certain control elements and not the overall design as well will have an impact. It's possible that the control elements would have been more visible on a website where the rest of the aesthetic complements them.