It will fail if input raster is stored in FGDBR format (geodatabase), either executing Region Group in ArcMap/ArcCatalog 10.7.1 or ArcGIS Pro 2.4.
If input raster is stored in ESRI grid format (in a workspace), it wil run like a charm. It took ~118hr to identify ~20 million groups from a raster having 14,951x12,704 8bit cells...
This also works fine in ArcGIS Pro 2.4 if your input data is a TIFF image. However, the performance will be a bit slower (~120.5hr) .