Assassin's Creed Unity was not a good game when it came out initially but it was subject of one of the positive review bombings on Steam. Valve talked about it recently, admitting the event confused them on whether to mark it off-topic.
Valve opened up about their bizarre situation that came to be after their introduction of the system that would mark review bombing off-topic in certain situations and the subsequent positive review bombing of Assassin's Creed Unity.
The company made the system in order to prevent tarnishing reputation of games whose developers and publishers pulled controversial moves somewhere else, which infuriated the players who would then decide to leave negative reviews for the title as a way of showing their discontent.
Borderlands and Borderlands 2 are great examples of the system working as intended after enraged fans started leaving negative reviews following Gearbox Software's decision to go with Epic Games exclusivity for Borderlands 3. Those reviews were marked as off-topic and they don't lower the review score of those games unless a Steam user specifically turns the system off.
Valve didn't take positive review bombing into account though. Following the Notre Dame fire, Ubisoft made Assassin's Creed Unity free for a week on top of donating €500,000 for renovations. The game featured the landmark and has the best representation of the church in virtual media.
It has been a month since those events and Valve have finally spoken about the whole situation with the surge of positive reviews. Granted, many were inspired by Ubisoft's actions but as Valve pointed out, most of the reviews actually focused on the way Assassin's Creed Unity was fixed over time and is very much a superior version of its former self.
Therefore, Valve decided not to mark the new reviews as off-topic but it took them a long time to work it out. They did their thoughts a whole month after the whole thing happened, after all.
As it turns out, many people still bought Assassin's Creed Unity on Steam in that week, despite being able to claim it for free on uPlay.
We have filtered out Valve's thoughts on the matter and posted a more concise version. If you feel like reading a major wall of text, you can check out the official post on positive review bombs.