Interesting topic!
From other responses, and watching the telecast, my opinion is its a combination of pitch (ride height sensitivity), and yaw sensitivity.
The stiff suspension leads me to believe the Williams suffers a bit from pitch or platform sensitivity, needing to try and keep it all as still as possible, but I think it is more a transient turn in issue, where the front wheels are steered, and the car is in yaw.
On Saturday, both drivers binned it on turn in to fast corners, and Kubica's previous comments from his drives has confirmed that is where its lacking. Running the 2017 wing for no better result gives me clue to say it underneath the car, but the whole flow regime under there is so complex. I think the hard part is to find the culprit. It is was apparent in the wind tunnel or through CFD, I'm sure there could make new parts to fix it.
As an aside, Its interesting to see that McLaren have reverted to running lots of Flo Viz on their cars, to try and understand what is clearly a complex problem....