To be fair Codex and Claude Code don't have a lot of secret sauce either. This is not to say that the harness doesn't matter at all or that you can't do any better, but whenever they benchmark it always turns out that you don't lose that much with even a very minimal agent. If a different harness makes better sense for your workflow I don't see why you wouldn't switch (or make your own if you have the resources to do so).

I'm also sticking to first-party agents, but that's more because I feel no compelling reason to switch and CLI agents fit well with my spartan technology choices. YMMV.