Here’s how it works, so far as I can tell.
Was it a magic-driven nuke?
Kind of, yes. It clearly operated on nuclear principles, causing radiation poisoning and an explosion of light and heat. The words used for it (roughly translated as “be not”) clearly imply a conversion of matter (which “is”) to energy (which “is not”).
Can you target something other than yourself? Unknown, but I’d say probably not. It’s very clearly a suicide spell—Angela implies that to use it would certainly kill her. None of the people who knew it (Angela, Galbatorix, and its originator) considered using it in a non-suicidal manner.
This might seem surprising, but given that using magic becomes harder over distance, it seems entirely possible that targeting anything much further than one’s own self with this spell is entirely impossible. Further, for living creatures, magic is linked with their own biological energy stores…so there’s another potential connection there.
The spell is clearly not targeting the caster’s entire body, judging from the energy produced—my guess is that it simply can’t. That would take too much energy from the caster, and the spell would fail. It’s a much smaller portion of matters that’s being converted to energy. But probably the caster’s own body or something very close by.
Could anyone use it who knew the words? No. Those are very basic words in the Ancient Language, and undoubtedly many magicians have used them before. Words aren’t enough—most spells require you to have some idea of what you’re trying to do. You can twist words around to do unexpected things, cast spells by intent alone, and even very occasionally get things wrong when you have a certain intent but the grammar is “stronger,” but to get this effect you very clearly need to understand what you’re doing.
To use the phrase this way, you’d have to understand mass-energy equivalence at at least a basic level, and understand enough about the grammar of the ancient language to understand what it defines as “being.” That’s why the spell’s not in common currency among anyone with a modicum of understanding of the grammar of the Ancient Language.
You wouldn’t need the words, though for a silent spell, just the understanding of what you were trying to do.
Finally, there is clearly a minimum energy input for a given output. Galbatorix was a Rider with multiple Eldunari. The other user was an elf, and we don’t know what kind of tricks they might have been using. Angela is Angela, and we don’t know how large an explosion she could produce anyway. It’s quite unclear how big an explosion the average magician could produce. What we can say is that it would be big on the scale of the sort of warfare they’re engaged in.
Yes, this does make magicians more dangerous than commonly realized. And it definitely makes Nasuada’s position on the regulation of magic, which might have seemed unnecessarily harsh, seem a little more sensible. She was very much aware of this capability.
However, magicians are rare and magicians of any competence of even rarer. We’ve seen, what? Various elves (highly limited in numbers), Trianna, the Twins, Angela…and a bunch of incompetents, who likely wouldn’t be capable of this sort of magic in the first place (it’s very much not just about the complexity of the words). And then add in how many of these individuals would be willing to commit suicide for their cause, and the viability of this strategy becomes relatively low. But that said, not impossibly so.
It would only likely be a nuclear-powered suicide bomb in the hands of a few very powerful individuals, though—in all likelihood even those few who could work up the power to cast the spell at all could produce only relatively modest explosions. The main protection would be similar to that against modern suicide bombers and nuclear weapons—keep an eye on the few people who can do it, don’t let them near you unless they’re on your side, check everyone at the gate and at the door.
And no, this still isn’t as dangerous as the Name of Names. The latter allows one to control magic and spellcasters to an unprecedented extent, including the nuclear spells…making it much more dangerous.