If you are wondering about the flavor of this card, Nga (in the Nenets mythology) is the son of Num (the god of sky and heavens) and the god of death (thankfully, it's bringing death to the corp and not to you).

Under normal circumstances, Nga is kind of like a Time Bomb 4-6 :
Time Bomb:
* costs 1
* requires 1 run on centrals
* normally takes 3 turns minimum and maximum to deliver it's sabotage 3
* Rigging Up would make you lose 2 on install discount but speed up the doom clock by one turn
Nga:
* costs 2 and 1 MU (harder to acquire, especially for anarchs)
* requires 3 runs anywhere (if you want to, you can use Hannah "Wheels" Pilintra to trigger Nga without losing a )
* normally takes 3 turns minimum to deliver it's 3 * sabotage 1 (slightly weaker since it gives the corp more control over the trashed cards as they can draw in-between the milling, but also slightly stronger since sabotage can happen even on the turn of installation)
* Rigging Up would make you lose 1 on discount but add another turn of sabotaging

All of this, however, changes with charge. The most efficient killer ever and The Time Travel Trojan - both can infinitely recharge your Nga during its milling run, and turn it into an endless milling machine (no wonder they made Nga unique), whereas Time Bomb would get a faster doom clock at best... Just remember to leave at least 1 counter on it so it doesn't trash itself!

Overall, I think Time Bomb is better for runners who are not as concentrated on sabotaging, while Nga is better for runners who can get a little bit of setup to make it the true god of death (of course, running both is the best choice if you have the deck slots)

[written after the release of Liberation]