What is the continuity and replacement risk with a freelancer versus an agency?

This is the single biggest difference. A freelancer is a single point of failure: if they get sick, take a better offer, or simply disappear, your project stalls and the knowledge in their head leaves with them. Replacing them means finding, vetting, and onboarding someone new from scratch while your project waits. An agency carries continuity structurally: multiple people know your codebase, knowledge is documented, and if one developer leaves the agency replaces them internally, often with a 5 day replacement clause. For any multi month engagement, continuity risk is the strongest argument for an agency.