I just had Fable run overnight in a loop, and it fixed ~150 compiler crashing bugs that Opus had kept deferring.

I wouldn't start with Fable - when I use burndown loops I tend to include instructions to document progress and set aside anything that turns out to be harder than expected, and solve the easy stuff first. When a model runs out of easy stuff and start struggling to make progress on what is left, I can let it keep churning on that - they get there eventually - or I can bump it up to a smarter model if one is available.

Opus had churned a week driving down spec failures, and did a great job. The 150 Fable took overnight were the ones Opus had kept putting aside.