Also flexbox support in IE is still terrible, so for those of us building sites for the open-web it's a non-starter for things where a 'just doesn't work' fallback is unacceptable, such as the main site structure and alignment of core elements.
Currently I only find flexbox useful for things like vertical centering, where it's not the end of the world if the element doesn't center in IE.
Not to say it isn't frustrating, especially since it works extremely well in Edge but MS said they won't port the fixes over.
For contrast if you work against a framework like Bootstrap 3, sites 'just work' all the way back to IE8.
I've taken to using CSS frameworks as reference guides for solving certain problems, I tend to not use them directly.
But Bootstrap has a nice "out-of-the-box" experience. I know much about usability engineering and user experience, but not so much about graphic design. And bootstrap fills this hole for me. Getting something out fast that doesn't look like 1999