In a poly repo setup, agents are less effective having to infer changes across repo boundaries using specs rather than code as context. Changes that impact multiple repos are also much messier to wrangle.
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 What happens when feature branches get out of date and don't see the upstream change that breaks the local branch?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.