What happens to the singularity from a destroyed warbird? Freed from its containment, does it then anchor itself in space and therefore present a navigational hazard from that point on? Or does it gravitate towards the nearest gravity well (possibly causing the Hobus supernova, as someone suggested)? If the latter, wouldn't it be a death sentence to a planet to destroy a Romulan ship in orbit?
It might survive the destruction if it is self-sustaining, but then it really needed to be the mass of a star.
I don't think they hurl around super dense black holes with warp speed.
This is as much magic as other treknology (huh, my phone's spellchecker knows the word treknology! How awesome is that?).
Personally I'd say the singularity decays very fast once released.
The energy released then could be as dangerous as a warpcore breach, that's why they can't shut down the singularity once activated.
Antimatter warpcores look really practical against that.