Old Lightmass based on CPU is not so accurate as GPU version made by Luo.
GPU version gives much better results. For example if you have two walls side-by-side ( or one wall made from two different meshes ) then with CPU bake you could have two different shading (first can be brighter that the second )! With GPU Lightmass everything is coherent.
But still GPU LM gives some white splotches caused probably by bounced rays. (see attachment).
And shadows can give some strange patterns on surface.
Maybe it is when some parts of the surface that gives a bounce are more reflective that others and somehow diffuse is not nicely dispersed on the surface that takes the bounce - I do not know how to say - I am not pro with that matter.
Even that GPU is faster and better - more accurate at lightpropagation than old CPU version.