Fable 5.1 is good for better knowledge and tasks that require a more meticulous output, and it's objectively better at design and mockups IMO, but I rarely resort to it outside of that.

I generally use Fable for high-level blueprinting, architecture, and technical planning, and I tell it to write most of its documentation in a way Opus 5.5 will understand and maximally benefit from; with the added prompt for Opus to concede when a task would be better off to bring Fable back in. I've had ZERO problems.