I understand that the new PowerShell 6/core lacks support for Windows GUI libraries, I have developed some important projects in PS-5.1 using the Windows.Forms
thanks to all who added informative comments.
After a bit of research I discovered that the XAML UI is going to be available in the .NET Core, its support has been added the Universal Windows Platform and Xamarin applications, hence it will be available in PowerShell core/6 for GUI solution development.
Development in XAML appears to be very straight forwards, in fact, in some aspects - easier and quicker than other APIs (especially if you have to hand code the UI components).