I work for a company with about 1000+ engineers that run a Bazel + Git + GitHub monorepo. As with all workflows it has its painpoints, but I am quite fond of it. It only needs a team of 8 to maintain all the integrations and performance optimisations.

GitHub is taking the problems of monorepo user's seriously (https://github.blog/2021-03-16-improving-large-monorepo-perf...) and have been responsive to the bugs and poor performance that monorepo's typically cause and exercise.