I've been following the development of this for quite a while, and one feature that is not erm, featured, is that the actual emulator itself is composed of isolated modules. This means that it's possible to swap out say, the graphics implementation to do a high definition drawer, or substitute the processor with a plain Lua script - essentially, it turns the Gameboy itself into a library! This could be used to make higher-level ROM hacks and patches without needing to know the nitty-gritty of the whole system.