Thats not quite right. Being stable is subjective. Ubuntu and a TON of distro's are based on Debian Unstable. Does that mean they are unstable because of that?
Debian unstable and testing are a bit more unstable, of course, but they are still good enough for desktop.
Given how often Ubuntu breaks hilariously on dist-upgrades, yes, I would say they are unstable. Though for slightly different reasons than the ones the grandparent poster mentioned.
Debian unstable and testing are a bit more unstable, of course, but they are still good enough for desktop.