It's not even about being pixel perfect. I can't vertically align text with an image. If I align it with one font, the other one is misaligned. I need something like media query for fonts.
I don't see how flexbox solves this problem. Just because you vertically center text, doesn't mean that the visible text is actually centered. It depends on the font.
The CSS property leading-trim [1][2] is designed to solve this alignment problem (among others). However, it's still at the Working Draft stage, so it's going to take a while before browsers start supporting it.