Funny you talk about facts vs assumptions when all I see is the defenders of UWP saying "wait for this, wait for that, but in the meantime please don't say bad things about Microsoft and UWP".
So in your mind who is talking about the facts here and who is assuming?
I could go back and quote things. But I'm on mobile and at work and don't feel like it.
But you don't have to go far to see comments about how UWP is will to lock devs and users into the Windows store, how it is a threat to open Distribution, or how the QB rendering solution is because Remedy, due to UWP had to use the Xbox solution, or how KI's framerate bug is due to UWP.
There are specific issues that are due to the limitations of UWP (SLI, Vsync, modding, overlays etc). Only modding is one that is inherently problematic. There's no technical reason the others wont be fixed in OS updates.
There are other issues that are due to developer decisions or oversight during the porting process. Those should be obvious, but folks attribute that to UWP. The Quit button. The QB rendering solution on PC. The framerate issue in KI.
Then there are other issues, that are probably due to piss poor management. Like releasing these graphics intensive games prior to the OS being able to support many of the features gamers expect.
I really don't see much sentiment that suggests UWP is great in every way. But in a discussion about the shortcoming of these PC releases, it's important to discuss facts, rather that make blanket statements that aren't factually accurate.