why do I envision teleporting Flayed Ones, appearing to tear those Guardsmen each a new... Oh yeah, that's why. Still, I'd think it could be a fun opponent. On the plus? side, the Guardsmen's lack of armor makes Gauss a bit less terrible; Guardsmen die expecting to die. On the other hand, with the newer Necrons being more cravenly and personality-bearing (maybe not the grunt troops), there could still be some options for limited interactive role-playing.
Just please remember, whoever is running them, that Necrons can cheat. Where other foes have to fight through you, Necrons can just analyze your fortifications, and have Scarabs phase-dig under your Proteus Command bunker, and siege it, or bomb it. The most heavily fortified structure can be under attack with ease, crawling with Scarabs, Warriors, and/or Flayed Ones, and all of the outside guns and troops will be useless, as with all potential avenues of escape. In many cases, the Necrons get to pick their own fights. If Necrons were the foe in OW, rather than Chaos, secessionists thinking about Chaos, Eldar who are way too Chaos to actually not be Chaos, and Orks, LM Ghanzorik would likely already be dead, as would his four successors. If there is a ground-based target, and Necrons on the ground, the Necrons CAN overtake it.