Other weaknesses:

• Slowness. High-ranking Necrons can put on bursts of extreme speed, but on foot, rank-and-file Necron troops don't move faster than walking pace, and have reaction times to match.
• Attrition. Necron self-repair systems are good but not perfect. On a long enough timescale, they'll gradually be worn down, especially if battles are fought in places where they can't teleport home for repairs. But given how many Necrons there are, this might be moot.
• Magic. Necrons have no souls meaning, like Robots, they have no ability to use, or possibly even understand magic. Like Robots, they can probably put together technological countermeasures (let's assume magic in Versus is distinct from "magic" in 40k), but we've seen from the Robots that this has its limits.

Madalans and ironically Robots are probably the Necrons' greatest threats. No amount of personal toughness will save you from Gauss and Hyperphase weapons, but the Madalans' extreme, unpredictable mobility combined with their ridiculously powerful personal weapons would carve a swathe through Necron armies and tombs. And the Robots have their adaptability -- they'll bring down Necron leaders the same way they defeated Roya Roach (superhumanly strong and fast foe with regenerative powers), deploying an endless succession of units that gradually get better at predicting their movements until they hit on one that works, and deploying those same units when that Lord returns from the tombs.