High end VFX/CG usually tessellates geometry down to micropolygon, so you roughly have 1 quad (or two triangles) per pixel in terms of geometry density, so you can often have > 150,000,000 polys in a scene, along with per vertex primvars to control shading, and many textures (which can be paged fairly well with shade on hit).
Using ray tracing pretty much means having all that in memory at once (paging sucks in general of geo and accel structures, it's been tried in the past) so that intersection / traversal is fast.
Doing lookdev on individual assets (i.e. turntables) is one place where GPU rendering can be used as the memory requirements are much smaller, but only if the look you get is identical to the one you get using CPU rendering, which isn't always the case (some of the algorithms are hard to get working correctly on GPUs, i.e. volumetrics).
Renderman (the renderer Pixar use, and create in Seattle) isn't really GPU ready yet (they're attempting to release XPU this year I think).
I disagree with this takeaway. But full disclosure I’m biased: I work on OptiX. There is a reason Pixar and Arnold and Vray and most other major industry renderers are moving to the GPU, because the trends are clear and because it has recently become ‘worth it’. Many renderers are reporting factors of 2-10 for production scale scene rendering. (Here’s a good example: https://www.youtube.com/watch?v=ZlmRuR5MKmU) There definitely are tradeoffs, and you’ve accurately pointed out several of them - memory constraints, paging, micropolygons, etc. Yes, it does take a lot of engineering to make the best use of the GPU, but the scale of scenes in production with GPUs today is already firmly well past being limited to turntables, and the writing is on the wall - the trend is clearly moving toward GPU farms.
Isn’t there a better Vray or Arnold comparison somewhere?
As in my summary comment, an A100 can now run real scenes, but will cost you ~$10k per card. For $10k, you get a lot more threads from AMD.