Ubuntu Stable Release Updates
Stable release updates are automatically recommended to a very large
number of users, and so it is critically important to treat them with
great caution. Therefore, when updates are proposed, they must be
accompanied by a strong rationale and present a low risk of
Backports of new versions of KDE and major KDE apps for Kubuntu which
are either too large a change or not yet tested enough to go to Ubuntu
I think that the new version of the KDE is in the "too large category". The new backported KDE version is also build against the new version of the Qt.
Can I trust this PPA?
The packages are backported by the same Kubuntu developers who are building the Kubuntu 12.04 packages.
But there is always the risk ( https://help.ubuntu.com/community/UbuntuBackports ):
When using Backports, it is important to understand that there is an
inherent risk in backporting software. Although backported packages
are tested by the community before they are included in the
repository, there are very occasionally bad interactions with the
older software on your system that are overlooked...