To be fair, the Ubuntu quality assurance process seems somewhat shoddy. On stable releases, I've seen 2 rather serious issues with minor updates.

In one update, Ubuntu pretty much broke the PCI detection code in X, so that X always thought that there were no cards. Luckily, it was only a matter of waiting several hours for a fix, and updating from a text console.

In another update, the Canadian english localization completely broke KDE, making it segfault on startup. Again, wait a day or two and update from a text console.

Right now I'm on vanilla Debian (unstable branch) and I find that it has had less critical breakage than Ubuntu's mainline branch. I'm not sure how Ubuntu should update their QA process, though.