Margins of flex child elements won't collapse automatically like regular elements. It can be solved but it would be unnecessarily complex for the simplicity of the layout.
Because it's a just a simple stack. The intent is concistent margins between siblings within the wrapper. What came to mind were paragraphs. Or perhaps images. Or both?
If other browsers besides Firefox add support for the gap property, Flexbox will be perfect for this. The owl approach has some subtle issues, like not taking into account elements which show/hide based on breakpoint.
