> Another thing that doesn't seem to have been mentioned on this post is that raytracing can be accelerated massively (where applicable) by using instancing. A single object can be used many times in a scene, only differing in its transformation. This allows the geometry to be stored in ram once and re-used, incoming rays that are incident on an instance's bounding box can simply be transformed into the local space of the instance. Of course this is of no help in an extremely complex scene full of unique objects, but in practice you can make great savings (and create very complex scenes that are cheap to raytrace) this way.

You're also forgetting about the flip-side. A classic benefit of scanline renderers was that a scene could be split into multiple parts, which (via a z-buffer) could then be combined without any further rendering. A raytracer, on the other hand, has to have access to the whole scene (if you consider reflections, which make culling objects essentially impossible) to calculate any given pixel sample.

I don't know whether this is still an issue, but for a while it was a barrier to raytracing scenes on a Pixar level.