It seems that it is possible to use the CPU and the GPU to render simultaneously. In my case I have a AMD threadripper, so it has no graphics cores. Ticking both boxes, increases the CPU load but with a small test file (a lamp shade with 3 materials) I see no diference in render time.
On the plus side, it is at least 7x faster than using RhinoCycles: taking about 31s to do 1500 samples with ProRender vs 3minutes and 33s with RhinoCycles.
I’ve then tried ProRender with a larger file I worked recently (a building) and did renders with RhinoCycles. The project was originally started in Rhino6, had VisualArq objects (which I had to explode for rendering) and was subsequently moved to Rhino7. I had a few crashes that seem to be related with a previous version of ProRender that was installed in Rhino6. After removing all the previous ProRender version files. The file no longer crashes but it does not render either and the command line shows no error.