It seems like people only talk about Opus and never Fable anymore. I myself use Fable since it's supposed to be better or more powerful.
When I run multiple versions of a technical task and have Fable choose the best one blind, Fable itself always picks the Opus one. This may be due to the tendency of Opus to probe deeper and work longer (producing more evidence) though.
It feels like there is a big hole right now where Fable 5.5 should be. Opus 5.5 seems to work and explain better, yet it's supposedly limited compared to Fable.