Are those long unsupervised sessions useful? In the sense, do they produce useful code or do you throw most of it away?

I get very useful code from long sessions. It’s all about having a framework of clear documentation, a clear multi-step plan including validation against docs and critical code reviews, acceptance criteria, and closed-loop debugging (it can launch/restsart the app, control it, and monitor logs)

I am heavily involved in developing those, and then routinely let opus run overnight and have either flawless or nearly flawless product in the morning.