I don't have a lot of experience with monorepos but I've found them very useful for:

- Simple projects with a server and an SPA component - frontend and backend code for the same feature is on the same feature branch, can be tested and reviewed together.

- Projects with a couple of microservices that share some common libraries - these libs can be directly referenced by the microservices instead of being published on an internal package server. This of course has its drawbacks too but overall, the benefits have outweighed them.