My Necrons may not have played either, but they live in fear of Orks and Imperial Guard.

Both field a tonne of chaff, and that's what I find we struggle with. My army in particular excels at overwhelming firepower, but that only works against small armies. Orks in particular give me am extremely finite time to do my damage before they're in HTH. And then it just gets messy!

Guard? Sure I can drop tanks well enough, but again not fast enough. Any army which can out-attrition Necrons is onto a winner!