Where fable has blown me away is converting entire code bases and or refactoring across many different segments.
It’s far more careful than opus and puts far more effort into testing and validating by default.
Switching back to opus at work was a downgrade. Similar requests felt more clunky and needed far more hand holding.
Some of it feels boiled down to "opus works better when told not to be dumb, fable's prompt tells it not to be dumb."
If they know much of what the tool is used for, they can customize prompts to "do that usage right" even if the user doesn't know exactly how to ask for it.