Come sail away

[Moose]

I think it makes them dangerously sensitive to crosswinds

Do you have any evidence that it makes them more or less sensitive to cross winds?

If you're going to argue this without doing detailed aerodynamic modelling, I could make an arguement that it protects half of the rear wing against the effects of crosswinds, and hence makes the cars less sensitive to crosswinds, not more.

[strad]

really
I think it makes them more stable, in most situations, mid corner and keeps them from spinning much like the side of a WoO wing, so that it is a trade off. They are already sensitive to crosswinds this just makes it worse.
Ever drive a van in a crosswind? . I think it is self evident that more surface area is going to increase this sensitivity.
If you're going to say not, I'm willing to consider any quality model you'd care to present.

[garygph]

Due to the position of the "sail" being at the rear of the car you might find that it actually helps in crosswinds by steering the car into the crosswind. I am sure I heard Mr Newey say that it will help with that but I may be wrong. I do remember reading what I said in the first sentence but do not remember the publication offhand. If you really want I will look for the book and let you know which one it is.

[roon]

Yes. A stabilizer behind the center of gravity, like on aircraft. The two RW endplates will have a similar effect. That the endplates are smaller now may explain the shark fin's return.

[McMrocks]

We have also seen shark fins flexing during cornering quite a bit in the past

[Moose]

really
I think it makes them more stable, in most situations, mid corner and keeps them from spinning much like the side of a WoO wing, so that it is a trade off. They are already sensitive to crosswinds this just makes it worse.
Ever drive a van in a crosswind? . I think it is self evident that more surface area is going to increase this sensitivity.
If you're going to say not, I'm willing to consider any quality model you'd care to present.

You're the one making the claim, so it seems like you need to provide the model.

A van is quite a different beast from an F1 car, there's no downforce production being robbed by the crosswind, so it's not a good point of comparison at all.