[tomvbussel]

Mitsuba lacks a lot of features that are necessary for production: there is no proper support for hair (Mitsuba only has a simple Kajiya-Kay shader and no specialized acceleration structure for ray intersections), there is no support of subdivision surfaces, no support for displacement mapping, etc. All of these features are necessary in production.

Mitsuba is great for research though. It has support for a lot of experimental rendering algorithms (such as Metropolis Light Transport), which are not supported in production renderers. Due to Mitsuba's modular structure it's also incredibly easy to implement new algorithms, which makes Mitsuba a great framework for comparing rendering algorithms. A lot of papers use Mitsuba to compare their new algorithm to other algorithms.

Most production renderers use CPUs (just like Mitsuba). GPUs are incredibly powerful and some benchmarks show 400% speed improvements over CPU-based renderers. The problem is though is that GPU-based renderers struggle with the scenes that are used in production. Typical production scenes contain over 250GB of textures and over 10s of GBs of geometry (pre-tessellation). This isn't going to fit the in the memory of a GPU and the GPU will spend most of the time loading assets from the hard drive (or network).