Flexbox actually makes a developer's life a lot easier, so much so that its hard to find the things it lacks in.

That said, here are some things I tried to make with flex and failed:

1. Masonry layouts
2. Layout transitions like this:

Do you think it will be worth replacing bootstrap grid with flexbox and is there a framework that uses flex?

I'm almost only using bootstrap for the grid now a days, maybe a little bit on tables and panels too, but that's easy to make my own version to get rid of bootstrap for good, grids are the most crucial part at the moment.

Basically what I've found what Flexbox can do is a Card grid with different heights. Something like....

Don't really know what is not possible using flexbox.But I know what ww can make using flexbox. I have made some layouts using flexbox. You can check out moshfequr.com/flexbox-layouts.html

Make up for a lack of logical document structure, content of value, or accessible design concepts... in fact in a lot of cases it actively encourages the exact opposite of all three.

Though it can also be used when you have all three to force a layout the markup structure doesn't match -- assuming you can get Safari to play ball with it aging like milk so badly it's becoming the new IE6... what with the Apple rotting on the vine thanks to Google absconding with all the talent in forking off "Blink" from webkit.