My experience with Flexbox is that it's a struggle to use it on its own for responsive behavior but is much easier when combined with percentage-based width, height and margins.
See this as an example:
See this as an example:
For example sometimes you may want a 2-column layout only - so the items within the flexbox container only ever wrap once. Defining percentage width of those items, say 48% to allow for middle margin, is the only way I know to achieve this.
I've started using Flexbox after waiting a LONG time for it to become safely adopted. It's one of best things to happen to frontend dev in a long time. No excuses now not to make your own responsive template.