Cycles is pretty great. RenderMan's value is entirely in it's extremely fine level of control. It's typically used in production settings by RenderMan power users who can write custom shaders, complex interactions with non turn key systems, like custom simulators, and other production workflow tools.
It has been hammered on in many ways by many people for several decades and is focused on stable efficient production. For this reason it can take quite a lot of work to get great images out of it, the configurability and control come at a cost.
I think of light transport simulators as a completely different thing from production renderers. A simulator is like a camera that captures reality, where as production renderers are not bound by reality so they are more like fine art tools (brushes, rulers, etc.).
It has been hammered on in many ways by many people for several decades and is focused on stable efficient production. For this reason it can take quite a lot of work to get great images out of it, the configurability and control come at a cost.
I think of light transport simulators as a completely different thing from production renderers. A simulator is like a camera that captures reality, where as production renderers are not bound by reality so they are more like fine art tools (brushes, rulers, etc.).