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A few more points to consider (some have been touched on already):

-The majority of the damage done to the defenders is via airborne bio weapons that disseminate through the air and water, choking/crippling/killing the majority of life on the planet. Unfortunately those don't work very well against Necrons. Also, the attack has a heavy psychological element to it, which doesn't work well on the souless calculating Necrons.

-I think it could be possible to digest living metal, but the cost would be pretty significant. First, the majority of Necron forces (I'm thinking on the order of 90% or so) would phase out, then come back ready for more. Then the living metal would still be trying to repair itself, causing some serious heartburn.

-From what I understand, Tyranids don't do much 'mining'. Necron tombs are pretty deep underground, so I could even see a Necron Overlord just shut up the doors, and let the planet be consumed above. Only minor damage would happen to the Tomb. If the Necrons did fight back, the Tyranids would lose even more resources trying to invade a tomb (swarms would be funneled, the big bugs won't fit everywhere, and the tyranid "ships" couldn't bring any long range firepower to bear.