I wouldn't want to do it at Google's scale without Google's tooling. But my experience has been that, at the scales I've worked at (no more than a couple million LOC across the organization), the limiting factor isn't source control, it's the build system. Maven, for example, doesn't really understand monorepos, so it can be a bit difficult to figure out a how to implement a policy for deciding what needs to be built when that's less heavy-handed than, "build everything always."