Subspace weaponry - a plausible candidate.
'Insurrection' shows the potency of such weapons - and their danger: unpredictability.
'Serpents among the ruins' establishes that the romulans have such weapons and used them at least once before.
No it did not the ship that used the subspace weapon was who the Romulans were fighting, the Romulans also baned use of these weapons after the planet they were defending and it's possibly billions of inhabitants were destroyed by the weapon.
About repercussions resulting from the romulans using metaweapons:
All other powers already knew the romulans had metaweapons (subspace weapons, for example).
All other powers had metaweapons of their own (for the Federation - Genesis, Thalaon, TOS, TNG establish half a dozen more).
This means, of course, that no one should be alarmed about the romulans using metaweapons - and planing to invade Romulus or something alarmist like this.
There could be accusations of breaking the various treaties forbidding the use of such weapons - of course, considering the situation in 'Destiny', I doubt they'll be too loud.
and since the treaty banning them is the same one that keeps cloaks out of Federation hands, the feds get cloaks and subspace weapons so win win for them.