The first image is Cycles. I had to set the area light (directly above the cube) to 1000 W to achieve this level of brightness. At this point, if I switch to Radeon ProRender, it looks like the second image below. I had to change the light intensity to 100 mW to achieve this level of brightness like the third image.
My question is, why is the light intensity so drastically different? I mean, I could understand some slight differences, but this is 1000 W vs 100 mW, a 10,000 times difference.