Turing completeness. No, I'm referring to using Python (waf, conan, etc), Javascript (nodejs scripts, esbuild, etc), and other "real" programming languages.
It's incredibly frustrating to have to configure a build with something as shitty as YAML. There's a tangible amount of money I have wasted in organizations on it.
A build is not actually a static configuration of another system. It's a program and deserves everything we need from programming languages.