pro-3d / anti-flat: details made possible by volume add to the icon's rich design, making its personality stand out, while flat icons results in boring, uniform design in comparison.
pro-flat / anti-3d: the apparent uniformity makes even slight nuances readily apparent and efficient to parse, making each icon stand out against other flat icons, while 3d icons are full of complex and distracting artifacts.
Bottom line: don't mix both, as the result is that 3d crushes flat with its rich details while flat makes 3d look needlessly busy and noisy. Consistency is key to reducing cognitive friction.
pro-3d / anti-flat: details made possible by volume add to the icon's rich design, making its personality stand out, while flat icons results in boring, uniform design in comparison.
pro-flat / anti-3d: the apparent uniformity makes even slight nuances readily apparent and efficient to parse, making each icon stand out against other flat icons, while 3d icons are full of complex and distracting artifacts.
Bottom line: don't mix both, as the result is that 3d crushes flat with its rich details while flat makes 3d look needlessly busy and noisy. Consistency is key to reducing cognitive friction.