This would center a fixed sized element in absolute position to page, this was never an issue. Problems start when you do not know size of your element and its parent. Flex finally fixes[1] this issue but it was not fully supported until recently[2].
[1] https://philipwalton.github.io/solved-by-flexbox/demos/verti... [2] https://github.com/philipwalton/flexbugs
Most of the flexbugs are now fixed and flex should be recommended way of making layouts.