A lot of our decisions were based on our past experience with Bazel. We both have worked at companies that tried to use Bazel and failed miserably. There's stuff we like about Bazel (and copied in some capacity, like file groups), and definitely parts we hate about Bazel.

Bazel doesn't work well for every language, but for those languages that it does, it definitely makes sense to use Bazel. For those languages that work better with something more lightweight, that's where moon comes in. I'm assuming you work at Google, so you're experience around Bazel is probably much better than those that don't work at Google.

I appreciate all the feedback and comments though, very much appreciated.