Comment by bluGill

I disagree. CI and build systems have different responsibilities and so should be different systems. Both are extremely complex because they have to deal with the complex real world.

Many people have the idea they can make things simpler. Which is really easy because the basic problems are not that hard. Them someone needs "just one more small feature" which seems easy enough and it is - but the combination of everyone's small feature is complex.

Both systems end up having full programming languages because someone really needs that complexity for something weird - likely someone in your project. However don't abuse that power. 99% of what you need from both should be done in a declarative style that lets the system work and is simple. Just because you can do CI in the build system, or the build system's job with the CI system doesn't mean you should. Make sure you separate them.

You CI system should be a small set of entry points. "./do everything" should be your default. But maybe you need a "build", then "test part-a" and "test part-b" as separate. However those are all entry points that your CI system calls to your build system and they are things you can do locally. Can do locally doesn't mean you do - most of the time locally you should be an incremental build. Nothing should be allowed past CI without doing a full build from scratch just to make sure that works (this isn't saying your CI shouldn't do incremental builds for speed - just that it needs to do full rebuilds as well, and if full rebuild breaks you stop everyone until the full rebuild is fixed).