Which can't be the case, because VOY has showed us Romulans who were former drones.
Yes, but that’s most likely going to be ignored if it doesn’t jibe with the story they’re trying to tell. And for all we know, those Romulan Borg did indeed cause problems with whatever ship they were on.
Well the cube in question in Voyger that had a romulan on also suffered a massive submatrix collapse and was abandoned.
"The only Romulans assimilated" just became the new "Odo is the only shapeshifter" or "Data is the only android" of Trek.
To be fair, Hugh actually said they were the only Romulans assimilated to his knowledge, and didn't rule out others.
Also, as Crazeywok said, the Cube from Voyager that had Romulans was disabled. It's still possible that their presence was the reason for that. Orun might not have been like "the afflicted", but it's still possible that the condition was due to peculiarities with one or more of the individual Romulans from the Shaenor.
I can't comment on Romulan drones in First Contact, I only remember hearing about, and seeing Klingons being used for the first time.