Well, the stuff IS a very strong oxidizer.
But considering that the solid form is a hard white salt-like substance,
and it melts at 41 °C, and boils at 47 °C, so it can be a wee bit tricky to pump through an engine.
Add the fact that it quite happily decomposes down to NO2 + O2 at temperatures well below its freezing point, and you have one spectacularly uncooperative oxidizer.
N2O6 is so unstable, it decomposes before it can be detected.
Same for N3O6.
N4O6, more properly N(NO2)3, does exist in that it can be made in a lab, but it refuses to sit still in large enough quantities and for long enough for its physical properties to be examined. This does not bode well for its use in a rocket engine.
As a rough rule of thumb, the more N and O one crams into a single molecule, the more the molecule will resemble high explosives rather than usable rocket oxidizer.
Could I not interest you in more tractable oxidizers, like dioxygen difluoride, better known by the cuddly name of "FOOF"?
It should be more stable, and deliver better Isp than these higher order oxides of nitrogen. (And yet appeal to the masochistic mad scientist that wanted to play with high order nitrogen oxides.)