Fundamentally speaking, the key point is really just hermeticity and reliable caching. Running arbitrary commands is never the problem anyways. What makes gcc a blessed command but the compiler for my own language an "arbitrary" command anyways?

And in languages with insufficient abstraction power like C and Go, you often need to invoke a code generation tool to generate the sources; that's an extremely arbitrary command. These are just non-problems if you have hermetic builds and reliable caching.