I'm thinking Occam's Razor might be at work here...we're speculating about meta-super-ultra-Earth-shattering-kaboom!-weapons, when this is a more simple, perhaps more realistic, and more elegant solution that does not involve the other Quadrant powers feeling the need to invade Romulus to keep these metaweapons out of the hands of the Empire.
the Romulans are known to have these weapons, and it's widely known that they have these weapons, it could be that either the RSE is sufficiently stable in other ways for these weapons not to be an issue, or that it's seen as far too dangerous to go after the RSE on account of these weapons--compare Mao's China, or unfortunate North Korea now. If the Romulans are sufficiently subtle in their foreign dealings, they might not be that big of an issue.
(Given Romulan foreign dealings since 2364, that's expecting to much.)
The other possibility is that they might not be known to have these weapons, or maybe not widely known. North Korea tests its nuclear bombs only a few hundred kilometres from large Chinese cities (the Chinese don't seem to be happy about this), but the RSE has huge volumes of space. If the RSE's isolationism is sufficient and counter-intelligence good enough, it might be able to build up an arsenal without anyone knowing. Or, a portion of the RSE might be able to build an arsenal of metaweapons without many people within the RSE necessarily knowing. The Tal Shi'ar's offensive against the Founder's homeworld may have been as much a surprise to the Senate and the other branches of the Romulan polity as the Obsidian Order's was to the Cardassian Union.
It might just be that the Romulans are just better positioned, not only by virtue of mentality but because of the way their technical base is set up, to metaweaponize their existing military technologies. It's been said that if Japan wanted, its civilian nuclear and space technology was such that it could become a nuclear weapons state in a few months. The other Alpha/Beta Quadrant powers have the same technological ability as the Romulans, the singularity drive being the main factor unique to the Romulans, but maybe the Romulans just have somewhat unorthodox technical manuals and classes at Grand Fleet Academy for engineers and commanders? If we're going by the novelverse, the suicide attack on Coridan had metaweapon-like effects even if the technology used was well-understood by everyone around.