Are there limits to SSRS 2014 RDL files going over to SSRS 2008 (or 2012)?
SSRS Report Definition Language for SQL Server Reporting Services 2014 cannot be reliably imported into previous versions of SQL Server Reporting Services.

The Report Definition Schema files were modified in 2010; SSRS 2014 uses that version, which supports features that are not supported in the 2008 version of the schema.

The 2010 schema is at: http://schemas.microsoft.com/sqlserver/reporting/2010/01/reportdefinition/

The 2008 schema is at: http://schemas.microsoft.com/sqlserver/reporting/2010/01/reportdefinition/