A missile capable of warp would be a fairly natural thing for Starfleet to have, though - and warp capacity for Starfleet hardware is generally indicated by the presence of nacelles.
Cardassians, of course, needed no stinking nacelles for their warp-capable missile. Then again, their ships don't use nacelles, either.
I'm sort of fancying a compromise interpretation here: those things in "BoBW" could well be former small starships, of "corvette" or "patrol boat" size judging by how they aren't invisibly small against the multi-kilometer Cube - but converted into suicide drones by removal of crew and addition of explosives. We do see the exact same design up close in "New Ground", after all, serving as a test vessel for the warp soliton, and the detailing there suggests a relatively substantial craft or vessel. (Also note the cameo in the "Unification" junkyard!)