Users have reported issues where they couldn't delete some of the Piwik files (especially /tmp/*) as they are owned by the webserver.
We should provide a one button option to uninstall Piwik that would
The uninstall will not delete data in the Database. It can easily be removed manually, or another Piwik created in another database.
It is quite important to make sure users can safely and easily stop using Piwik or migrate to a new server without leaving a few files behind.
Actually just added a simple FAQ rather than adding it as a feature... Uninstall / Delete Piwik