A general list of unsupported tools in 64bit Background can be found here: http://resources.arcgis.com/en/help/main/10.2/index.html#/Background_Geoprocessing_64_bit/0021000000...Most of these tools don't run in regular 32bit Background either, with the exception being the graph tools.Theres probably a few other tools not in this list which don't run in background.You can check if a given tool will run in the background by checking its General Properties page from ArcMap/Catalog. If "Always run in Foreground" is checked, it will not run in the background.
