Then there was Longhorn, which failed more due to internal politics than due to technical issues, as Midori later has proven just to be killed by management, as decribed by Joe Duffy postmortem reports.
WinRT/UAP/UWP are built on top of improved COM, and .NET has a native personality for them, using AOT compiler for WinRT/UAP (MDIL) and UWP (.NET Native).
Most of the new UWP stuff is written in .NET Native, with C++ taking care of the UWP runtime infrastructure, and the DirectX composition engine (Visual Layer).
The majority of Windows UI team UWP and FluentUI demos are usually done in .NET Native.