There are a lot of variables that will all contribute to accuracy ...
Quality of pictures, good focus, low noise
Number of pictures and overlap of pictures
Accurate scaling information
I assume you will rebuild the member in a cad program ... machining from a scan will not give a very even surface. There are some cadcam systems what won't accept meshes.
If you need a faithful reproduction and the member is rough hewn so it does not have any truly flat surfaces, (though they may be smooth), its possible you might need to do some noise reduction as a post operation to using Photoscan. Then use something like Geomagic, TSplines, NPower to produce a nurbs surface if that is needed for the CNC.