The main problem is that there are three versions of syntax to remember, but nothing that a pre- or postprocessor can't fix. Other than that it's pretty easy.
I found this to be quite handy: css-tricks.com/snippets/css/a-guide-to-flexbox
I've been using it in production without a problem, although a good fallback is handy (display: table / table-cell, or float: left for example) for older IE versions. Yes, some developers still need to support IE8... ;)