I just upgraded from Piwik 2.x to Matomo 3.11.0. No problems encountered.
I noticed that the “Visits in Real-Time” widget is not showing page titles correctly when the original page title contains an HTML entity such as & or " . These entities are shown with their HTML entity name instead of the actual character they are supposed to represent. The old version of Piwik I upgraded from didn’t have this problem.
For example, if the page title contains “ABC & XYZ”, the widget displays this title as “ABC & XYZ” instead of “ABC & XYZ” the way the browsers do.