How do I write workout programs for clients with complex needs?

You can program for the everyday client, which means your foundation is solid. The complex cases feel different because there is less room for error and no clean template. That caution you feel is actually good judgment; it means you respect the stakes. Break the hard cases into parts. What can this body safely do, what is the one goal that matters most, what must you work around? Start conservative, watch how they respond, and adjust. You do not need the perfect plan on day one. You need a safe first step and honest feedback. One small step: for your next tricky client, write down their limitations before their goals. Design within the guardrails first, and the program gets simpler.