They work great when you have many teams working alongside each other within the same product.

It helps immensely with having consistent quality, structure, shared code, review practices, CI/CD....etc

The downside is that you essentially need an entire platform engineering team just to set up and maintain the monorepo, tooling, custom scripts, custom workflows....etc that support all the additional needs a monorepo and it's users have. Something that would never be a problem on a single repository like the list of pull requests maybe something that needs custom processes and workflows for in a monorepo due to the volume of changes.

(Ofc small mono repos don't require you to have a full team doing maintenance and platform engineering. But often you'll still find yourself dedicating an entire FTE worth of time towards it)

It's similar to microservices in that monorepo is a solution to scaling an organizational problem, not a solution to scaling a technology problem. It will create new problems that you have to solve that you would not have had to solve before. And that solution requires additional work to be effective and ergonomic. If those ergonomic and consistency issues aren't being solved then it will just devolve over time into a mess.