Monorepos come with a lot of pain too. Two sides of the same coin. I manage the build system for a large monorepo. Questions that will get you to a primary source of pain...

How do you minimally build based on the changeset? How do you know this is sufficient for correctness? What happens when feature branches get out of date and don't see the upstream change that breaks the local branch? How do you version subprojects, as they change or as a whole?

Monorepos have a habit of creating hidden dependencies. The languages you use can help or hurt here.