FYI- I just updated the MR which supports extraction of regions into separate vtkPolyData and/or vtkPartitionedDataSet (which is a collection of vtkPolyData, one per labeled region). It’s threaded and quite fast.
There are two ways to do this: by setting the output style and optionally using AddSelectedLabel(); or by invoking ExtractRegion() or vtkExtractRegions() (these latter two methods do not cause filter reexecution). Note that there are options to extract just cells on the boundary of their region (i.e., touching the background label/region), and/or to clean the region’s points (i.e., points and cells are renumbered so that only points used by the regions cells are produced on output).
Look at the test TestSurfaceNets3D4.py so see how this works.