Flexbox is nice and makes some things easier, but its still (IMHO) a half assed solution compared to something like QML's anchors.

In QML I can center like this:

    anchors.centerIn: parent

(replace parent with id of item you want to center in if not the parent item). You can also do just vertical or horizontal centering, or you can fill another item or you can anchor eg the left of your item the the right of another item and the right of your item someplace else and it resizes/repositions your item as required.

Flexbox still often requires lots of nesting to achieve what should be (and is in QML) relatively simple layouts. I've also hit a few things that I couldn't do at all with Flexbox (although that could just be lack of CSS/flexbox skill on my part - but to that I say that CSS gets very hard quite fast, while complex layouts in QML are pretty easy).