I found tonight how to make the plug-in work under Windows with its version 1.0 Beta 3 (running Sweet Home 3D either with the Jar executable, or with the free and paid installer versions).
Actually, I had to rebuild Yafaray with the latest version of MSYS2, and use more recent versions of the DLLs it depends on. I'll try the same rebuild under Linux in the coming days.

YafaRay is not using more than 32% of my CPU capacity, whereas Sunflow is using 98%.

It's strange, it's not what I see on my iMac, where YafaRay and SunFlow use as much CPU power as possible. I wonder where it could come from...

Just out of curiosity: Why go for another CPU-renderer and not a CPU/GPU-renderer?

There are not so much open source candidates that could be tested. From last year study, YafaRay seemed a better candidate even if OSPRay which uses the GPU too seemed interesting too.