I've seen the case of Sencha Facebook app brought up many times now, but I've never seen someone point out their huge flaw in the scrolling. At one point in the demo, they trivially dismiss the fact that they made the scrolling inertia smaller to give the data a chance to load; but that's in my opinion THE main flaw of the app. As a matter of fact, that's the first thing I notice on every web app I've seen to date: the quirky scrolling physics. For me, Sencha's implementation falls right in that uncanny valley and it kills the UX of the app each time for me. I don't see why they put so much emphasis on the loading speed or smoothness (not that it's not important), when all this effort to improve the UX is completely offset by the scrolling.

They had to do more work, relatively speaking, to make that work well than it would've taken to do the same with a native app.

Sencha's little demonstration was the very definition of "bending over backwards".