Hi, thanks for posting!
This isn't a framework I'm familiar with myself (you're the first person to ask about it), although a quick analysis suggests that the framework requires some kind of integrated interceptor to execute the assertions before completing the test.
I'm not yet sure what's involved in introducing the integration point that can make this happen. A big constraint at the moment around NCrunch's Xunit support is that it's current handled through Gallio, which does have its advantages, but also introduces certain limitations. I think that this is something that may need to be addressed in future when NCrunch's Xunit support is matured (i.e. direct integration).
Cheers,
Remco