Ubuntu tests upgrades between LTS releases extensively and so do Ubuntu community developers and advanced users.
As you said, server users always upgrade. Enterprise desktop users upgrade. A lot of general users don't - they reinstall.
The upgrade process you should use with Ubuntu is different to Debian. In Debian you'd use apt, in Ubuntu you should use the update manager. Update manager specifically checks for upgradability, it has specific warnings and workarounds that are added for each release following testing and reports from end-users. This makes it much easier to upgrade.
The reason why users think the Ubuntu upgrade process is less reliable compared to Debian is they serve different user segments. Most Debian users are technically advanced, engage with the distribution and they often stay in their stream (e.g. testing). Ubuntu is used by lots of normal users - many may not pay particular attention to how packaging works - this means they often do things that make upgrades complex: install community PPA's that advance packages past the next LTS version, install Node into /usr by wiping out apts package knowledge, etc, etc. The range is just bigger.
For many normal users upgrading may no longer be the best option - it's just faster to reinstall. I personally do upgrades, I still find them magical and it's part of the fun: http://www.futurile.net/resources/ubuntu-upgrades/
As you said, server users always upgrade. Enterprise desktop users upgrade. A lot of general users don't - they reinstall.
The upgrade process you should use with Ubuntu is different to Debian. In Debian you'd use apt, in Ubuntu you should use the update manager. Update manager specifically checks for upgradability, it has specific warnings and workarounds that are added for each release following testing and reports from end-users. This makes it much easier to upgrade.
The reason why users think the Ubuntu upgrade process is less reliable compared to Debian is they serve different user segments. Most Debian users are technically advanced, engage with the distribution and they often stay in their stream (e.g. testing). Ubuntu is used by lots of normal users - many may not pay particular attention to how packaging works - this means they often do things that make upgrades complex: install community PPA's that advance packages past the next LTS version, install Node into /usr by wiping out apts package knowledge, etc, etc. The range is just bigger.
For many normal users upgrading may no longer be the best option - it's just faster to reinstall. I personally do upgrades, I still find them magical and it's part of the fun: http://www.futurile.net/resources/ubuntu-upgrades/