To answer 'what is the technical reason', I took a look through the tracker and GitHub notes. The issue seems to be two-fold - a failure to delete certain files during the upgrade, and failure to apply certain SQL updates. However, if only affects some and not others. See the bug tracker for additional info.
The general best practice option approach seems to be:
If not already borked...
- Backup!
- Disable
System -> Remember Me
plugin - Do the update
- Enable
System -> Remember Me
plugin
If borked and no backup...
When the install fails on url /administrator/index.php?option=com_joomlaupdate&task=update.finalise with php error '\plugins\system\remember\remember.php on line 94'
Running \cli\deletefiles.php from the command line then refreshing the /administrator/index.php?option=com_joomlaupdate&task=update.finalise gives FULL SUCCESS
This commit ostensibly fixes the issue, but suggest it's to do with $this->app
not being prepopulated by the system. It may also be worth a look if the above fails. Keep in mind this is quite a recent pull request (16th April 2014) and hasn't been approved in the tracker: YMMV.