How big of a monorepo are peeps thinking of? What use cases to optimize for?
How long does your team want to wait to run your builds and tests?
For js projects, the pre merge build-validation chain may not so bad, but one should consider how often you need to make sweeping breaking changes.
Mitigations to minimize disruption from bulk of breaking changes (e.g. versioned contracts/implementations) can mean you preserve a fast inner loop.