The Amazon poly-repo setup is an engineering marvel, and a usability nightmare, and doesn't even solve all the major documented problems of poly-repos. The "version set" idea was probably revolutionary when it was invented, but everyone I know who has ever worked at amazon has casually mentioned that their team has at least one college-hire working 25%+ time on keeping their dependency tree building.
This really shouldn't be the case as of about 5 years ago, a massive effort was done to get all version sets merging from live regularly and things were much healthier after that. For what it's worth I suspect the usability of Brazil before then was still on par or better than the usability of a unkempt monorepo (which is unfortunately all too common).
