PHP7 is a large change to the PHP language, and while it does provide a 2x improvment improvement in performance, we can not stress enough the importance of testing your whole site/application when upgrading. You should essentially regards regard it as a re-launch.
- While we try our best to cover all possible cases, especially in eZ Platform, our newest version. Our QA testing does not cover all the possible use case you might be using eZ Publish for.
- Extensions made by the community and shared on http://projects.ez.no/ or other places (github/composer, ..) might or might not have tested on PHP 7 yet.
- Your own code base will need to be adapted for PHP 7.