Problem is that there's no issue at all. XCB is a core part of modern X11 ecosystem. Any X application uses it either through Xlib API or directly, and direct usage, exposing asynchronous nature of X11, can help to reduce latencies .
If he really knows, he intentionally spreads the FUD. XCB is not "crippled" neither is it "wrapper" for any "higher level API". It's true that it does not cover all Xlib functionality, but:he seems to actually know what he's talking about
1. Using XCB does NOT imply not using Xlib. They are fully interoperable and can be used together (as Qt developers do, and they do use GLX). XCB needs to be used where it can bring speed up
2. More APIs become available in XCB with each new release of X.org
And, of course, XCB has nothing to do with KDE and NVidia breakages.