Which makes your choice of Fable Legends being a proof of that bizarre, because Fable Legends is an Unreal Engine 4 title.
Like, UWA has literally nothing to do with it.
You know what makes things easier? Standard implementations. SDKs. APIs provided to you. That is what MS has done in the UWP. If it was super easy for everyone to implement without MS APIs, you would see cross saves for pc and consoles more often. Parties and messaging going cross platform. But you don't. Rocket League as an example. Those two communities are only linked by an ability to match make. One could say that MS should make these things available to all devs, but then we are getting back to wanting to push their store and their own services.