Interested to hear what flexbox features were unsupported? I’ve been in a same kind of situation but was able to support everything on “IE version” of the app by using flexbox.
Most of flexbox is supported in IE11, but some "defaults" are not the same as other browsers --> see the known issues tab if you're interested, most of the time it works but when it doesn't it's frustrating https://caniuse.com/#feat=flexbox
Its been too long for me to remember the details but I remember flexbox 90% working but having some bugs that were a huge pain to work around. Its at the point where you can use flexbox on IE but you have to specifically test each use on IE to make sure it works.