Depends on your project. My experience is that if you use a single language throughout, then your language-specific build tool probably suffices. But once you go polyglot, interactions between these language-specific build tools are bad and you'd be better off replacing all of them with one tool that understands everything. Especially if you have cross-language dependencies (e.g. your Go program depends on C++ using cgo, or your Python has a module written in C). Whether or not you want that tool to be Bazel is your choice.