It is important to remember that fearless means units auto-pass Moral tests, but Leadership tests still have to be taken. In the case of the Necrons this would apply to things like charging a C'tan, for which a Leadership test must be passed or no charge can be made. Fearless does not provide any help in these situations.
As darklove says, Moral tests and LD tests are not the same thing, so Fearless doesn't protect against all LD based tests. It's important to remember this especially for Fearless Psykers, who have to pass a LD test to use their powers on LD 7 if they're affected by Soulless.
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