While I've never run Nether Void in Pox, I've run it in a Mono Black Stax deck running Chalice, Sphere's, and 2xNether Void. I brought it to a few tournaments and after going 1-3, 2-2, 2-2 etc. over and over I gave up trying to make it work, it felt almost "too cool" but just wasn't paying off for me in THAT deck (not saying at all that someone couldn't make it work in another , aka Pox or w/e).
You said black Standstill, or black CounterTop... Imho, it's more like the former, since it affects both players and in that it can be inconsistent and if you're +Board Position, its awesome and you ride it to victory (nothings cooler than having a 3sphere+Void in play and beating them with a factory I'll tell you). But like so many times, if they got a Goyf or something up on you, it's the opposite having it in your hand. I'd simply not be able to play it till I'd remove their threat, then they play another threat their turn, etc.
Nether Void is such a tough card to work with. From what I've seen, it either needs to come out early in some sorta all-in control deck and stop them right away (Chalice, Unmask, sphere's) or it comes out later in a control deck, like Pox or MBC to lock them out once you've gotten the advantage. The ladder is probably better.
I threw one in my SB, not really 'cause I wanted to just because I could. But to be honest the one problem I have with it is Trinisphere works so much better under LD, because under Sinkhole's and waste it plays much more like the black Countertop because it allows you to lock them out AND be able to play your spells if you stay above 3 land, whereas Nether Void is symmetrical.. you have to be GUARANTEED the better board position before you considering playing whereas 3sphere you can drop even if they have a goyf out, if your say at 3 land and they're at 2, and if you were to topdeck Sinkholes under Nether Void you need 5 mana till you can play it.

