Working on a UWP title for Unity, we've run into several obscure issues that may help some other people out. First off, here are the full Xbox Live plugins (grab the linked .unitypackage):
There's a helpful link in there about setting up more detailed Unity features for UWP and Xbox Live. The biggest thing there for us was the note about setting the "InternetClient" capability in the UWP publish settings inside Unity PlayerSettings. Without this, trying to unlock achievements will give you a cryptic error message: "The application does not have the cloud notification capability"
Also useful is getting multi-user support, which will allow you to support the user picker on appropriate platforms. Specifically, you have to edit the Package.appxmanifest file after building your UWP solution to add "<uap:SupportedUsers>multiple</uap:SupportedUsers>" to the Properties section. It's briefly mentioned in the "Building and Testing UWP" section of this page.