The documentation includes a comparison chart including this:
Fail– xUnit.net alternative:Assert.True(false, "message")
(It doesn’t show Assert.Pass, and I’ve never used that myself, but I suspect the alternative is just to return from the test. Of course that doesn’t help if you want to throw it in a nested method call. My suspicion is that it’s not very frequently used in NUnit, hence its absence in the comparison chart.)