Actually - it's the relative user-bases and the complexity of the user-cases. An APT based system is capable of a system upgrade - but you have to help the sys

Actually - it's the relative user-bases and the complexity of the user-cases. An APT based system is capable of a system upgrade - but you have to help the system. The previous commenter is completely right that one of the underlying assumptions is that the package database is clean. This applies to both Ubuntu and Debian. The main challenges for Ubuntu is that there is a lot of ISV software out there which can break as it's not part of the distribution [0], end-users tend to run a variety of PPA's [1] and in general they don't realise the importance of the package database[2] and the classes of hardware that Ubuntu is used on means that driver upgrades can be complex. Actually, if you use Ubuntu's documented upgrade process it runs a variety of checks which standard apt doesn't. I suppose that with fast Net connections and cheap disks upgrades aren't as needed as they used to be - but they're still one of the best bits of 'magic' I enjoy from the Ubuntu/Debian world. [0] The counter-point being that end-users want that software e.g. Skype [1] PPA's are great for end-users as they get to mix stable system software with the latest 'apps' but they cause problems for upgrades because you can't possibly test the variety of upgrade paths [1] You can argue that they shouldn't have to - but that's a different matter.