I would definitely recommend to use Flexbox, it's a great tool, saves a lot of time. You can solve complex UI problems with just a few CSS properties ! It's worth the small 4% that won't have the support for it ! Plus, I do really think that Flexbox is gonna be the new standard if it's no already.
Thanks for your answer. The combination of flexbox and CSS grid seems really promising indeed. I just wonder how long it will take for browsers to implement it (IE, I'm looking at you) and for users to update to the latest versions.