So I gave it a try, and here are my results in a fairly complex scene (using a bunch of custom area lights that are basically spotlight arrays, so it had a ton of data to work with)
CPU vs GPU Lightmass - Imgsli - CPU vs GPU baking results, using “UltraHigh” on the GPU lightmass and very high settings on the CPU to eliminate any and all inconsistencies between modular components (production, quality 4, smoothness 0.25, and scale 0.1).
As you can see, the GPU lightmass had… problems. Namely in the bounce lighting, it’s way too dark.
CPU vs GPU Lightmass 2 - Imgsli This is the same test, but with “LowerQuality” on the GPU for the sake of time, and changing Diffuse Boost to 1.5 in the world settings. As you can see, it’s significantly closer in the end result. I suspect the extra color might just be from boosting the diffuse like that, it’s the brightness that needed to be fixed.
Also worth noting, the “FastPreview” preset would stall at 93.33% in the scene, eventually taking longer than UltraHigh did on that one mesh so I canceled it. This happened both times when testing that file.
As a side note, just something I encountered when using base lightmass, not sure if this is something that can even be changed, but is there any way to make it look at world position offset or tessellation when calculating the baked shadows? It’s a pretty fringe edge case I ran into the problem, but when vertex painting a mesh’s heightmap with a ton of displacement it would be pretty significant for catching the shadows properly. Again, not sure if that’s a thing that can be solved with this, just figure I may as well ask.