It's just you. Fable 5 is consistently better than Opus 4.8 at literally every single level, for me. (I always use both at xhigh, for reference.) I could go down a laundry list of various issues I have with Opus that I don't have with Fable. For me, Opus 4.8 < GPT-5.6 Sol < Fable 5.
Plus Fable is way less annoying to talk to than Opus 4.8. Opus 4.8's writing style is absolutely insufferable. Fable has some of the same quirks but it's way less bad.
We see no difference between opus 4.8 and fable except that fable is slower. So +1 for not just GP. We use neither interactive, just via our own tooling so the ‘talk to’ doesn’t apply and we use it 24/7. We currently run 25% of tasks on both fable and opus and the rest only opus. The 25% are being code reviewed side by side and we do detect when fable switches to opus for ‘security concerns’ nonsense. Opus generally finishes sooner, results are similar quality.