Hi Tim,
My understanding is that the UWP platform is quite similar in structure to the original Windows Store platform. Certainly, it is the spiritual successor.
The platform itself is tightly locked down at the O/S/Win32 level, which makes certain activities difficult or even impossible at runtime. A Windows Store application can only be launched within a strictly controlled sandbox, and the manner in which the sandbox is created is indirect and not well documented. It's a whole new completely closed platform that is designed not to be tampered with. In other words, it's a horrible nightmare to integrate with. The original integration with the Windows Store platform under NCrunch took months to implement and required massive workarounds resulting in seriously degraded performance. The integration point was eventually abandoned after the platform design changed in a way that made continuous testing basically impossible (see
http://blog.ncrunch.net/post/Windows-Store-Apps-No-Longer-Supported-By-NCrunch.aspx for more information).
Because MSTest/VSTest is developed by the same company that builds the platform, they have their own internal integration that essentially backdoors the sandbox. My assumption is that ReSharper invokes MSTest via standard APIs and MSTest handles the sandboxing. This approach just doesn't work for NCrunch, as NCrunch needs much more control over the runtime environment to enable its data collection.
So basically, it's hard. Hard to tie into, hard to get it running fast enough, and also very hard to maintain it. It's not impossible. But it would involve pushing the rest of NCrunch's development back and many other features would need to be shelved to make room for it. Given my previous history with trying to integrate with closed platforms, I would need to see massive demand for this before I could justify the effort.
You may wish to look into placing your backend code in portable class libraries. My understanding is that many people are doing this to work around the sandbox limitations when unit testing.
