I disliked everything about that rule - the way it's usable on turn 1 so almost always going to be on turn 1 (might as well do it when the table is full), the ability to use it before the opponent's first move meaning it's more likely to cause morale failures to kill units near board edges, the hits being on a full third of all enemy units (how hard is it to get within 48"? the table isn't that big), the unavoidability of it, the lack of clarity about which facing of a vehicle it hits, the lack of clarity about cover saves and the direction of attack, the way its power varies from extremely weak (like Knights could care!) to very powerful (an army with many units of 3 henchmen?) in a way completely divorced from the cost of the necron unit causing it, and finally, the random allocation rules taking a long time to actually do correctly for large units (we've already rolled a dice for every single unit, and then for the amount of hits, and then for how many wounds, but now which 4 of those 50 guardsmen took the hit? let's start rollin', I've no other plans this weekend).
It's not a game breaker or anything like that, I'm not even going as far as to say it's overpowered for his cost. It's just the sort of rules writing that makes me go GAAAAAAAAAH!!
Just offering a second opinion.