Flexbox is kinda at that point where it’s teetering on the edge of being “mainstream.” It can be used most places, with the exception of IE. Even so, it has a somewhat steep learning curve and a lot of its basic functionality can be accomplished in other ways, often easier (e.g.,
display:table layouts and inline-block grids).
If you want compatibility, go with other solutions. If not, go for flex. Either way, it is something you should be getting exposure to with the goal of figuring it out as time goes on.