As others mentioned, asset sizes are one problem. I'm currently involved in a production where render nodes got upgraded to 128GB RAM, since 64GB wasn't enough.

As others mentioned, asset sizes are one problem. I'm currently involved in a production where render nodes got upgraded to 128GB RAM, since 64GB wasn't enough. Also, path tracing may superficially at first glance look like a good fit for the GPU, but once you look closer, it becomes less so. With incoherent rays and surface shaders, memory access quickly becomes the bottleneck and the GPU cores stall. There are approaches to improving that (batching and sorting), but those can add quite some complexity. Out-of-core memory for GPU rendering can allow for scene sizes to exceed VRAM, but then memory access becomes even slower. Again, with sorting and moving things to VRAM on demand there are paths to reduce the impact of it all, but those are again not trivial given the incoherent access pattern of simulating millions random walks. [source: I am a developer working on the Cycles render engine (Blender) for a production company.]