one of the biggest differences between buck and bazel is,

when you want to write new build rules(to support your specific use case or to support an unsupported language etc):

- in buck you should update the buck code itself(you should touch so many places in JAVA) - whereas in bazel, you can extend bazel with starlark(subset of python) language and do all sorts of things like supporting whole new language. without touching even a tiny bit into bazel core.

This was the main reason my company is switching to bazel from buck