For completeness I thought I should add this:
It's a plot hole.
The Romulan Orum was assimilated prior to Seven's liberation from the collective, in Voyager Unity. Additionally, in Infinite Regress, one of the personalities that emerges from Seven was a Romulan, indicating that at least one other was assimilated.
It's worth noting that the page for the ...
It's unclear what really happened in the Neutral Zone, and if the Borg actually assimilated anyone
It might be a plot hole, but there really is no sufficient information on what happened. This is what we know:
The outposts weren't simply destroyed, they where "scooped off the planet", which is something we haven't seen on screen yet:
DATA: Captain, ...