Mono repos come with a particular challenge: If you have five projects {A,B,C,D,E} in the same mono-repo, you definitely do not want to be building B,C,D and E every-time someone commits code to project A! This is unimportant at small scales, but as the team grows, building and continuously deploying 'all the things' on every commit just doesn't work out.

So the first naive solution is say "we can enumerate all the things that need to be built for project A". This rapidly breaks down when someone figures out they can abstract a shared dependency for A,B and D into some other part of the mono-repo.

So now we enter build dependency tools, first Make then some other flavour, then we jump straight to Bazel because someone read about it in a Google publication, then to some custom build scripting because Bazel didn't do this hyper-specific workflow thing someone wanted... In the end maintaining a mono-repo build process becomes a hyper-specialised job function that is almost always kicked to the wayside.

In small companies, fewer developers than you can count on your fingers, or truly huge Googles that have their own VCS flavour, it can be shown to work well: But I have yet to hear a story of something in the middle succeeding with a mono-repo.