In my personal experience, all problems you will experience can be fixed by changing document structure (sometimes you need an extra container for example) and writing custom css, I’ve managed to support internet explorer 9 and up, as well as most other browsers, if you are interested I can search for the exact versions of each browser that I actually support. All browsers below those versions won’t work with ionic and are a pain to manually support, but don’t really worry, except for ie (which below 9 isn’t really used much anyways) those browser versions are rarely used.
I’ve gotten some problems with internet explorer 9 and centering content (i.e., where ionic will center the content out of the box, it didnt’work on ie9), and of course some spacing here and there. Also “fixed” footer/header had some problems, but nothing we can’t handle
As for flex-box like things, you just write some custom styles and you’re done I don’t come across it very often.
Also, for older versions of IE, make sure you’ve got the development vm’s microsoft provided for IE development The “emulate older version” idea of IE really sucks, doesn’t work “like” the real versions at all.