[matheusmoreira]

Unity builds are useful too but they have limitations. They are equivalent to full rebuilds and can't be done in parallel. The optimizations they enable can also be achieved via link time optimization. Language features that leverage file scope can interact badly with this type of build. They require a lot of memory since the compiler reads and processes the entire source code of the project and its dependencies.

Unity builds improve compilation times because the preprocessor and compiler is invoked only once. It is most useful in projects with lots of huge dependencies that require the inclusion of complex headers. The effect is less pronounced in simpler projects and they shouldn't be necessary at all in languages that have an actual module system instead of a preprocessor: Rust, Zig.