The problem is challenging foremost on the level of architecture. Designing an API is never easy, and attempting to expose GPU-level parallelism to a language as high-level as Lua requires a great deal of care to be usable while still being useful. Creating such an API requires significant knowledge of the abstraction layers between Lua, C, and CUDA as well as knowledge of the typical use cases for high-level parallelism.
My partner and I know neither Terra nor LLVM (which Terra compiles to), so creating these high-level bindings requires a great deal of initial investment. The existing interface between Terra and CUDA is sketchy at best, so we will need to implement significant new functionality into Terra in order for the Circle Renderer to function properly.