Indeed, but raytracing data structures aren't complex. Primary rays scream on GPUs, they cache amazingly. However, secondary rays for GI/FG and similar are prob

Indeed, but raytracing data structures aren't complex. Primary rays scream on GPUs, they cache amazingly. However, secondary rays for GI/FG and similar are problematic since they are trashing the cache and are branching like mad. So classic Whitted ray tracing is mad fast on GPUs, but add global illumination or similar and you've got a problem. Not to mention memory issues others have mentioned. Octane, Brigade, Keyshot are top GPU renderers these days - all very limited to what you can do with classic offline renderers which are dominant now like mental ray, vray, arnold, renderman...