I haven't had any trouble doing flexbox for IE11. I always make sure to test in IE11 because of the caniuse warnings; I have very rarely run into any problem with IE11, maybe once or twice, and they were easy to work around problems.
I wouldn't be scared of using flexbox even if you need to support IE11. I'm not sure where caniuse's dire warnings come from. (I have run into different flexbox behavior between FF/Chrome about as often I have run into flexbox problems with IE11; I generally couldn't tell you which of FF or Chrome is correct/incorrect in those cases).
I've in the past done flexbox without too much trouble even on IE10, but I am not myself concerned with IE10 anymore.
I wouldn't be scared of using flexbox even if you need to support IE11. I'm not sure where caniuse's dire warnings come from. (I have run into different flexbox behavior between FF/Chrome about as often I have run into flexbox problems with IE11; I generally couldn't tell you which of FF or Chrome is correct/incorrect in those cases).
I've in the past done flexbox without too much trouble even on IE10, but I am not myself concerned with IE10 anymore.