Vertical centering has been relatively easy since CSS 2.1 (Table model); flexbox of course offers much more than that. Flexbox may be finalized and unchangeable, if only because so many browsers already support it unprefixed (as support for such properties is almost never removed) – however, like much of the CSS3, flexible box model is not yet a W3C Recommendation, and is, in theory at least, subject to change. With proper tools, a prefix here and there is not a huge burden.

The reason why many browsers rush to unprefix CSS3 is not that it's perfect, but because of the lazy web developers who only prefix stuff for WebKit. This may or may not bite the community in the future.