Looking at their app I don't see it as a generic thing at all. They're aiming for consumer-oriented UX... unless you're doing something 100% custom and wild, then you will look out of place on your respective platform using generic toolkits.... or you'll typically have to re-invent the wheel for each platform in some non-native toolkit.
They're also on iOS first. IMHO, UIKit (and soon SwiftUI) is the best mobile development toolkit out there, including generic solutions.
FWIW the Facebook example is not necessarily generalisable. The original Facebook app was a thin wrapper around the existing mobile web site. It was a leaky and poorly designed abstraction that was fundamentally flawed.
So far the only actual success I’ve seen is electron on the desktop, and in that world everyone complains how apps use huge amounts of RAM.
There’s only one true success for a generic solution across all platforms: the web. Because it doesn’t pretend to integrate natively, nor depend on native APIs. But with it will always come limitations that cannot be avoided.