"On the other hand, you can upgrade to a new release as painless as a regular update on Debian/Ubuntu."
This is certainly not true.
https://wiki.ubuntu.com/PrecisePango...ure-1.Upgrades lists three known issues with upgrading to Pangolin. I would be very surprised if there are not more than that.
http://ubuntuforums.org/showthread.php?t=1973408 is not a painless upgrade.
http://ubuntuforums.org/showthread.php?t=1969817 is not a painless upgrade.
http://ubuntuforums.org/showthread.php?t=2043559 is not a painless upgrade.
http://ubuntuforums.org/showthread.php?t=1976791 is not a painless upgrade. I could go on, but you take my point. That's just from a casual Google for '12.04 upgrade site:ubuntuforums.org'; all those results are on the first page.
The fact is that an OS upgrade is a psychotically complex operation with an effectively infinite number of variables, it's as bad as or worse than the problem space for hardware support. I'd state that it's literally impossible for a general-purpose OS to guarantee trouble-free upgrades from one major version to the next. Ubuntu does not guarantee it in theory or in practice. Fedora does not. RHEL does not. Windows does not. OS X does not. Nobody does. It is not possible to do so.