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