Hello, I am the engineer in question. I am not actually super familiar with the details of the build system, but from when I saw, the main issues were:

- Lots of constant-time slowness at the beginning and end of the build

- Dubious parallelism, especially with unified builds

- Cargo being Cargo

Overall it mostly looks like a soup of `make` calls with no particular rhyme or reason. It's a far cry from the ninja example the OP showed in his post.