As many of you I'm really amazed by the new flexbox functionality. It saves a ton of time "coding to the middle". Thus, I've went through some sites to undo all the manual positioning and changed it to flexbox. Yes!
BUT, there's something you shouldn't forget; interactions!
Many interaction that we use to show a formerly hidden field [display:none;] use the [display:block;] in steps in their functionality. That means, if you change the site to use flexbox, interactions will not work (properly) anymore.
And enjoy a beautifully centered modal, pop-up, payment form and so on...!