That's like your opinion, dude...
The main thing limiting C++ code in UWP is sandbox, not the compiler. C++/CX is a glue language between proper C++ and the rest of the OS - you can freely mix it with proper C++ (11, 14, etc) within the same binary. I successfully compiled many open source cross-platform libraries for UWP without any serious issues.
As for complexity and generally APIs, UWP is way better than old Windows APIs. It is fairly consistent, predictable, and modern. I'm guessing you are really used to the old ways, but I don't really understand how can one prefer WndProc / manual message handling or message maps to clean approach of XAML; or think it was better.