Well, we saw what this wave of hurt did to a planet: it tore apart the physical structure, not in a flash, but in a push sustained for at least several seconds. This does not appear an unlikely result within the system whose star blows. But farther out, some sort of a radiation front would obviously be the real problem, even if only after a few years and with precise advance warning on when it arrives.
That would be the (much slower) physical blast wave of expanding stellar matter. the thing is, by that point planet that's being torn and blast apart is already a sterile rock.
A neutrino/gamma burst travels at light speed and in accordance with the laws of thermodynaics probably reaches a LOT farther out than the much slower moving ejected matter.
Yet in Trek and reality alike, the statistics must be considered: total devastation doesn't cover sizable parts of the galaxy, or Earth would have been sterilized several times over by now. What is the real lethal range of a neutrino burst? That is, in terms of inverse square, if the magnitude of the explosion isn't a major factor?
Timo Saloniemi
I didn't say it would cover a sizable chunk of the galaxy, but the radiation burst would sterilise every system within a few dozen light-years of Romulus. Given how centralised power, authority, information, and general logistics tend to be in an empire, that's a mortal blow to one of the Alpha/Beta quadrant's three main powers. Literally cutting it's heart out.
The evacuation effort alone would be crippling since aside from all of the disruption of personnel, the near total commitment of resources, the imperial fleet is likely stretched beyond the breaking point, so good luck holding onto planets that don't want to be under imperial authority anymore.
Add to that the refugee crisis and utter chaos that such a colossal power vacuum would cause; yeah, I'd call that a threat to all of the known galaxy. At least, to all intents and purposes from the Federation's POV.
IIRC neutrinos don't interact with normal matter, so the neutrino burst wouldn't do anything to anyone.
Watch the video, Kyle explains the science a lot better than I could. Short version: neutrinos DO interact with matter (how else could we detect them) but only just barely. However the sheer number of them created by a supernova is so mindbogglingly huge that they'd deliver a lethal dose of radiation to everything on Earth with in 1/20th of a second...then spend the rest of that second reducing it all to ash.
Rough analogy: nominal background neutrino radiation (the kind that's passing though us all right now like a ghostly blizzard of particles) is like getting a single drop of rain falling on your head once every 78 years. During a supernova, it's like the entire Atlantic ocean gets dump directly on your head, all at once. Squish.