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.
My understanding is that Facebook doesn't do Go, so it's always been a second class citizen for Buck. Go folks at Uber were spending a significant amount of time (in the scale of several months) trying to upstream fixes to Buck core, but found that Bazel extensibility allowed them to get similar functionality in a matter of weeks or even days. Buck also didn't seem to have a good story around Thrift/Proto and JS/node.
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.
