There is a key difference between parallelisation in xUnit vs NCrunch: xUnit runs tests in parallel within the same process (i.e. different thread), where NCrunch runs tests in parallel in different processes (i.e. different process).

This means that a test that cannot be parallelised in xUnit may still be parallelisable using NCrunch. For example, if a test is making use of code that isn't thread-safe and is making use of static state, it can't be parallelised using xUnit, but can still be parallelised using NCrunch. If a test is making use of a shared resource on the file system, it cannot be parallelised by either.

This makes it unwise for NCrunch to simply assume that a test fixture marked with an xUnit collection cannot be parallelised. Really, these are two completely different mechanisms and the only way for NCrunch to know whether the test should have parallelisation restrictions in place for cross-process parallelisation is through specialised attributes targeted towards NCrunch (i.e. ExclusivelyUsesAttribute).