Even if you're working on a large complex system like that, I believe coding agents are still useful at at least taking highly specific prompts/instructions you write and doing the writing for you. Then doing other tedious tangential work like generating unit tests over a pure function, adding comments, generating documentation, etc that all increase the quality of the codebase without requiring toil on your part.

With especially novel or complex projects, you'd probably not expect to use the agent to do much of the scaffolding or architecting, and more of the tedium.