The simplification of user interfaces has been proceeding quickly now that the last vestiges of skewmorphism are gone. Like any new technology, the first iterations of the interface must be familiar to the users. Early cars looked like carriages, early lightbulbs behaved like gaslight, early televisions looked like radios, and the first home computers worked like typerwriters (and still do!).
But any design trend ultimately overshoots the mark, in this case iconography has possibly become oversimlified, and buttons without outlines or contrast fill are being used because retina-class displays support the fine line widths.
Curt Arledge addresses one basic question in this user interface direction: does an outline or contrast button have more usability. Check out his results here
In summary, what seems to matter are two things. First, the users’ familiarity with the icon type: i.e. the common language all interfaces share to a great degree in the iconography alphabet. Second, that user testing is still required, since differences appear in counter-intuitive places, and some design decisions affect usability less than expected.