The argument has been stated that the IG has shown its power over the UN itself. That in a direct confrontation there is on panel proof that the IG overcame the power of the UN. Using
Here we have Epoch tell Quasar that using the UN is to be nullified.
Quasar is told by Deathurge that use of the UN is tantamount to suicide in the following two scans:
Here we have Quasar himself admit he has no idea how to use the ultimate nullifier
Here we have scans showing Thanos thinks well of the bravery of Quasar, but considers him a fool.
Here we see the Magus with the IG as it is able to be used in unison. That the word ultimate carries little weight in this current contest of titans. In this shot, the UN hasn't been fired yet. And that with a thought magus turns the weapon on its user... how can that be if it hasn't been fired yet... can magus really get quasar to fire upon himself?
If you look on panel you see the energy around the citadel which happens when the gems are lit up with power. After Magus attains the full power of the IG, or what he believes is its full power, Quasar fires the UN... but in doing so he slays only himself... something..
The fact is that noone believed Quasar to be smart enough to actually use the UN on Magus. Epoch, Deathurge and Thanos all believed that Quasar would die in the attempt. At worst we see that Quasar has in fact died in the attempt. The IG held all gems except reality, which means it could not affect the physics of the world or the UN itself. What other gems did it have, mind, soul, time, space and Power. Everyone believed Quasar didn't have the mental fortitude to use the UN on Magus, hence you have at best an example of Magus using the soul or mind gem to weaken Quasar's resolve enough to have him fail to fire then gun. Quasar had always hoped he would find a way to not die in the process... but in the end he does anyways. He died without being useful.