What if you have a microservice system with a repo-per-service setup, where to add functionality to a FE site you would have to edit code in three or four specific repos (FE site repo + backend service repo + API-client npm package repo + API gateway repo) out of hundreds of total repos?
Codebuff works on a local directory level, so it technically doesn't have to be a monorepo (though full disclaimer: our codebase is a monorepo and that's where we use it most). Most important thing is to make sure you have the projects in the same root directory so you can access them together. I've used it in a setup with two different repos in the same folder. That said, it might warn you that there's not .git folder at the root level when this happens.