AFAIK large part of FFB weight depends on grip itself, so with street tyres that loose grip quickly past the optimum slip angle we feel the understeer clearly.
With slicks we loose less grip, and for me it feels like ACC went to 11 with reducing grip drop at higher angles. And IMHO that's why cars feel so playful, having little grip dropoff allows the setups to be balanced more aggressively than in AC and still remain stable / well behaved.
Now since FFB feel heavily depends on grip, less grip loss in understeer situations creates less FFB change. So to me it is down to tyre model physics.
I don't think there are any special FFB effects (besides the ones in game options) in both AC and ACC, so if somebody feels FFB is better in one, it also means that basic tyre physics are better in that one. Meaning that if it feels like ACC is right about car feel, it is also right about FFB and vice versa.