The head of Epic Games, Tim Sweeney, doesn't like UWP too much, at least as it exists today (and for noble reasons). He will not support the new software (app) platform unless Microsoft makes some clear changes that guarantee perpetual openness. There really isn't anything, technically or legally, to prevent Microsoft (or an entity with authority over Microsoft, like governments, activists groups who petition government, and so forth) from undoing their changes going forward. If Microsoft drops support for Win32, apart from applications that are converted using Project Centennial or something, their catalog would be tiny.
SteamOS would kick its butt levels of tiny, let alone OSX, Android, and countless others.
As a result, Microsoft keeps it around, despite its unruliness. Functionality that is required by legitimate software make it difficult to prevent malware, and, even without an infection, it can make the system just get junked up over time.
UWP, on the other hand, is slimmer, contained, and authenticated with keys. This is theoretically easier to maintain, but at the expense of user control and freedom; freedom to develop and install software anonymously and without oversight. The first iteration was with Windows RT, which was basically iOS, right down to the “you cannot ship a web browser unless it is a reskin of Internet Explorer ((replace that for Safari in iOS' case))” and “content above ESRB M and PEGI 16 are banned from the OS” levels of control.
Since then, content guidelines have increased, sideloading has been added, and so forth. That said, unlike the technical hurdles of Win32, there's nothing to prevent Microsoft from, in the future, saying “Okay, we have enough software for lock in. Sideloading is being removed in Windows 10 version 2810” or something. I doubt that the current administration wants to do this, especially executives like Phil Spencer, but their unwillingness to make it impossible to be done in the future is frustrating. This could be a few clauses in the EULA that make it easy for users to sue Microsoft if a feature is changed, and/or some chunks of code that breaks compatibility if certain openness features are removed.
Some people complain that he wasn't this concerned about iOS, but he already said that it was a bad decision in hindsight. Apple waved a shiny device around, and it took a few years for developers to think “Wait a minute, what did I just sign away?” iOS is, indeed, just as bad as UWP could turn into, if not worse.
Remember folks, once you build a tool for censorship, they will come. They may also have very different beliefs about what should be allowed or disallowed than you do. This is scary stuff, albeit based on good intentions.
That rant aside, Microsoft's Advanced Technology Group (ATG) has produced a fork of Unreal Engine 4, which builds UWP content. It is based upon Unreal Engine 4.12, and they have apparently merged changes up to version 4.12.5. This makes sense, of course, because that version is required to use Visual Studio 2015 Update 3.
If you want to make a game in Unreal Engine 4 for the UWP platform, then you might be able to use Microsoft's version. That said, it is provided without warranty, and there might be some bugs that cropped up, which Epic Games will probably not help with. I somehow doubt that Microsoft will have a dedicated team that merges all fixes going forward, and I don't think this will change Tim's mind (although concrete limitations that guarantee openness might…). Use at your own risk, I guess, especially if you don't care about potentially missing out on whatever is added for 4.13 and on (unless you add it yourself).
The fork is available on Microsoft's ATG GitHub, with lots of uppercase typing.