Buck and monorepo is a disaster (there). A bunch of people from Google forced that stuff down everyone's throat. Concerns and real life problems were simply discarded. Another solution in search of a problem. Not invented here at the extreme.
At Uber, the Java stack is still sort of ok w/ Buck (because Buck handles some things better there), so they're taking a wait-and-see approach until the Bazel ecosystem catches up w/ Buck on the concerns they care about. But long term, we are envisioning implementing a company-wide monorepo, and that sort of entails having a unified build system.