I've not played necrons very often, only 5 or so times, but they aren't fun at all. A challenge I can deal with, necrons are not a challenge they're over the top. Last game I played resulted in me setting up my entire army, shooting the nightbringer for a turn, then 2 monoliths dropped into my lines and I spent the rest of the game taking troops off the board. I think I took out 3 warriors and wounded the nightbringer in total.
Against a unit of warriors with a lord w/res orb, and close enough to a monolith it takes one of the following to kill 1 single warrior:
72 lasguns [1 divided by (1 shot /2 to hit, /3 to wound, /3 to die, /2 WBB, /2 monolith)]
12 heavy bolters (36 shots)
9.6 missile launchers/lascannons
approx 3 battlecannons (assuming 5 are hit by blast, and a scatter is a miss (1/3 hit))
Its not the fact they can kill me easily, because who can't? Its the fact I can do sod all in return. And saying 'go for phase out' isn't an option most the time, because decent players keep their warriors out the way and let their unkillable monoliths do the work because who needs to do anything else if they can win against guard that way? Its the sensible thing to do. And looking at the stats above its not much of an option anyway.
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Don't get me wrong, I'm not going to flat out refuse to play necrons. But you must see my point?
[edit]
To destroy a monolith I'd need 18 veterans with meltaguns, who if they fail will get killed with the resulting lash/whip thing. So essentially with my 6 vet melta I have a 1/3 chance of downing it or another way, a 1/3 chance of possibly winning match. If the meltas even get close to it that is...