Flexbox is incredibly useful and should be the backbone of any webpage structure. Using the same flexbox classes over and over should be code efficient, easier and faster for development. A great balance.
I developed Flex Framework in a few hours and feel like other people should use it. It's a micro-framework. For the size of the CSS file, it's well worth it as an add-on for your CSS development toolkit. I wanted to put it on Github for this reason.
CSS
Flex really is only one dimensional.
Grid is for two dimensional.
I have added Flex Framework and Stooge CSS to my base code structure which is ITCSS. This system is a great way to do CSS and Flex Framework is an added tool.
None really. It is a very small library and I did in less than a couple hours yet it's so useful. However, the responsive part of it may not be as useful.