Scaling CI/CD Pipelines: Self-Hosted Runners vs. Cloud-Managed Services?

We're looking at ways to improve the performance and cost-efficiency of our CI/CD pipelines. Currently, we use a mix of cloud-managed runners, but as our team and codebase grow, we're seeing increased queue times and costs. I'm curious about experiences with self-hosted runners, especially for large monorepos or specialized build requirements. What are the key considerations for moving to self-hosted, beyond just cost? Any surprising challenges or benefits you've encountered when comparing them to services like GitHub Actions or GitLab CI's managed offerings?