IE11 doesn't have full support, so unless you're lucky enough to be able to ignore IE, you have to resort to jacks and workarounds to use grid. At which point it's generally easier just to use flexbox.

Grid will be awesome very soon.

Flexbox is for "1D layouts"; "lists" (including ones that wrap). Grid is for "2D layouts". In most cases you don't really need Grid (and it has a much steeper learning curve), but if you want to, say, mimic a <table> without an actual <table> element, Flexbox simply cannot do that.

I find flexbox has a small, easy to learn and teach api that allows you to do almost any kind of web app layout that you would need to do, exceptions being grids and tables.

Css Grid is less intuitive but they do make grid gutters much much easier.

Most of the time I reach for flex box. Only when I'm doing a guttered grid would I reach for css grids.

With how hard it is to get a companies UI devs on the same technology and patterns, flexbox is an easy standard to coordinate around.

Not to mention it's better supported.

Grid is awesome, and I highly recommend learning what it can do for you. That said, they serve different purposes. However, between the two of them they address 97% of my needs on the frontend. This[1] is a good rundown of what the two do/dont do.

[1] - https://www.youtube.com/watch?v=hs3piaN4b5I

Partial support in IE refers to supporting an older version of the specification.

https://caniuse.com/#feat=css-grid