Yes, good point. It was discussing in past test in the context of "DeCOMification" mainly. It would be interesting to have some insight into new component creation vs use of existing components.
Well it is just plain old COM, with a new base interface IIspectable that inherits from IUnknown and using .NET metadata instead of TLB files.
Using it from C++/CX is quite painless compared with the old ways, as it is quite similar to .NET, Delphi and C++ Builder ways of using COM.
For a little pain ATL style, you can use Windows Runtime Library.
Kenny Kerr and his team are pushing for C++/WinRT, a new C++ projection (language binding), based on pure Modern C++, with the goal to eventually replace C++/CX. Currently WIP, though.
For those that really like to suffer, the old way of using MIDL compiler with C is still possible for UWP applications.
From our projects, I would say most UWP developers use .NET for UWP controls and only resort to C++/CX if integration is C++ code is required.
COM usage on Windows only increased after Windows Vista and is at the heart how UWP works and user space drivers.