Well . . . That's kinda the point, Seth-Ra. Every army has things that are effective, but if you swap armies, you'll find that the "cheese" has weaknesses, and that your usual army (the one your opponent is now playing) has stuff just as bad.
Except with the Necrons. My friend cried cheese when my Eldar beat his robots for the fifth time in a row. So we swapped for a game, and I stomped him horribly. I've played with Eldar, Space Marines, Chaos, Tau, Tyranids, Orks, and now Necrons. Necrons are cheap. They are the most boring, obvious powergamer faction in the game. Don't get me wrong, I like playing against them.
The difference is this. Playing (either with or against) Eldar, Dark Eldar, Imperial Guard, Orks, or Tyranids, it's always a good and entertaining game, whether I win or lose. Against Necrons (and to a lesser extent, Space Marines), I enjoy killing them. That's it. Just makes for a magnificently boring game.