Bazel is a rats nest of complexity. In my experience it does what it does very well once set up, but setting it up is tremendously complicated, much of it IMO incidental complexity.

Rolling out Bazel at my prior employer tool about one person decade of engineering time. I've talked to other companies that tried to roll it out and failed. Bazel is hard

And Bazel is not really getting any easier! Like most projects, it is getting more complex over the years as features acrete. I think there is space for a tool like Mill for less sophisticated users who can't afford to spend a person-decade rolling out their build tool

There's a big difference between the complexity of setting up Bazel and the experience of using Bazel at a company where Bazel has been set up for you. As a user I love Bazel. When working with Java, I use the java_binary and java_library rules. When coding in Go, there's nothing new to learn regarding the build, just use go_binary and go_library instead. Everything is repeatable, builds and tests are cached, it's easy to query the build dependency tree, etc.

A few startups are offering "Bazel build/test as a service." It's one way to eliminate the work involved in setting up Bazel for an organization.