What all things are not supported in Salesforce Joined Reports?

The following items aren’t supported in joined reports, and aren’t converted:
Bucket fields
Cross filters
The Rows to Display filter

Some of the limitations of Joined Reports on dashboards include:

The only items on the dashboard that can be modified are the ‘Display Units’ and ‘Drill down To’ (a very cool, often overlooked feature that allows a Dashboard Component to link to another Dashboard instead of a source report).

• Joined reports in dashboard view can’t be filtered based on data.
• Dashboards that only have Joined reports can’t have a filter added.
• Just because a field contains the same data across objects does not mean they are guaranteed to allow grouping across blocks.  For example, if a lookup field to account name in a managed package has a different label than Account, grouping may not be possible.
• Joined reports require that the new user interface theme be enabled. Users without the new theme are unable to create, edit, or run joined reports. See Enable New User Interface Theme.