When I estimate Roughness, using QGIS [Raster > Analysis > DEM (Terrain models)], what is the unit of the values I get?
Pressing the Help button takes you to the tool's documentation on the GDAL homepage:
This command outputs a single-band raster with values computed from the elevation. Roughness is the largest inter-cell difference of a central pixel and its surrounding cell, as defined in Wilson et al (2007, Marine Geodesy 30:3-35).
Sounds like the units are same as the input raster value units.