Full disclosure: I work at Microsoft on XAML UI in C++. It's nice. Opinions are my own.
I believe that using Win32 API in C/++/# (maybe one day with Go if all the OS functions are mapped) is much more natural than forcing yourself to do it in JS. A lot of the Windows APIs still require you to manage some memory (I'm not sure how much is abstracted in C#, but if you are doing it through DllImport, Win32 will still expect you to free memory if you are using OS API functions as it was designed for C).
On top of that, you will spend a lot of effort trying to box/re-package documentation intended for the C/++ (ATL/MFC/WTL/etc) or C# (WPF) audience. This is probably the greatest cost. In the end, I suspect NativeScript code looking a lot like the C equivalents. Consuming third party libraries written in unmanaged code is likely to produce very ugly code.
I tried using RubyMotion a long time ago for iOS development. All of the sample code I found was in Objective-C, and I pretty much spent most of my time rewriting the code in Ruby, or parsing docs in ObjC and figuring out the Ruby way to do things. In the end, it was simpler to just do it in ObjC.
Writing the app in a SDK with a lot more support is just much better. Your sanity will thank you. And if you are writing cross-platform code, C has endured the test of time.
However, I've never had to work on cross-platform apps, and have never been subjected to JavaScript professionally, so my opinion is probably worthless.
