SenTestCase_Macros.hreveals its dirty secret in the comments:
Generates a failure when a1 is not equal to a2. This test is for C scalars, structs and unions.
So it's fine for ints and so on, but not a simple workaround for the cumbersome-looking string equality test [string isEqualTo:@"hi!"]. Unfortunately, it doesn't warn you that you're using inappropriate arguments; that realisation is a couple of minutes investigation away.