Star Trek: Picard has changed the history of the Romulans and the Borg in a decidedly unexpected way. In the Picard season 1 episode “The End is the Beginning,” Hugh - an ex-Borg drone - tells Dr. Asha that the Romulans on the Artifact (a disconnected Borg cube) are the only known Romulans to have ever been assimilated into the Borg Collective. However, this runs contrary to what has been established previously in a couple of Star Trek series.
The problem stems from a plot device in the Star Trek: The Next Generation season 1 episode “The Neutral Zone.” During the course of this episode, it is established that an unknown entity had caused the destruction of several Romulan outposts along the border of the Neutral Zone. As a consequence of this attack, many of their citizens were found dead or missing. Although this was a subplot within the episode, and the identity of the attacker was not established, it formed the base development for the most powerful enemies in the history of Star Trek. The attacks remained unresolved until years later, where this adversary would finally be confirmed to be the Borg.
This resolution to “The Neutral Zone” episode subplot began with the discovery of a Borg Footprint, identified during the events of Star Trek: The Next Generation season 4 episode “The Best of Both Worlds.” Just prior to Captain Picard’s assimilation, this technological footprint enabled the crew of the USS Enterprise-D to identify a Borg attack–confirming that the Borg did destroy the Romulan outposts. This subplot was concluded in the Star Trek: Voyager season 3 episode “Unity,” when a liberated Romulan Borg drone named Orum conclusively confirmed that Romulans had been abducted and assimilated far earlier than the events depicted in Star Trek: Picard.
Beyond this episodic evidence, there is potentially one further contradiction to Hugh’s claim. Although not explicitly stated on-screen, a species' numerical designation appears to be incremental, with the higher number suggesting that species was assimilated by the Borg later. The Borg designation for Romulans is species 3783, and for humans, it is species 5618. If these designations reflect the chronological experience of the Borg, then might it also confirm that Romulans were assimilated into the Borg collective earlier than humans?
This potentially makes sense, as the writers of Star Trek: Picard could have purposely changed Romulan and Borg histories to give Professor Ramdha’s assimilation a greater meaning within the context of the show - although it is also possible this retconning was simply just an oversight on their part. Certainly, though, it is very clear that the Romulans on the Artifact are not the only known Romulans ever to be assimilated by the Borg Collective.