Monorepos are excellent if you have a group of related projects, especially if they depend on each other. Instead of opening a series of PRs for each project in a specific dependent order, you can open a single PR and move everything forward in lock-step, ensuring that every commit / tag has all projects in the monorepo working together.

The biggest downsides are really around tooling: we've had tons of issues with our continuous integration environment (Travis CI) and build times trying to build only specific sub-projects using Lerna. The Github Pull Reviews and Issues pages can get pretty messy too, though we have a process to automatically add Github tags based on changes via Lerna.

I suspect this is why bigger companies seem to be more successful with monorepos than smaller ones, because they have the resources to invest in building their own tooling.