Well, it could be that by the time of the Dominion War (100+ years later) the Romulans had refined their devices to the point where it no longer was as big a threat as when they first began using them.This obviously doesn't happen, even with conventional warp cores, otherwise the Dominion War battlefields like Chin'Toka would be littered with tiny black holes or massive explosions.
I guess when they lose containment the worst thing that happens is the singularity collapses. They're artificial, so presumably the Romulans would bake in some safety features. They aren't Klingons!
The BOP in TOS did appear to completely destroy itself at the end of "The Balance of Power" leaving nothing behind for the Enterprise to find.
Also, the Enterprise-D's run-away warp core explosion in "GENERATIONS" did essentially Completely Annihilate the secondary hull and probably would have done the same to the primary hull if it had still been attached.
As far as the drives leaving "Black Holes" behind, I would surmise that the heart of the drive is most likely just a 'pin-prick' in size, thus completely disappearing after its initial release and wave of destruction.
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