This is not the first game to feature Yasuke as a samurai and not even first game where he is playable in some form, neither of which by itself ever bothered me. Koei Tecmo had him as a boss in NiOh 1 and one of many playable characters in Samurai Warriors 5 afterall, and that's a Japanese company.
But Ubisoft's marketing of the game presented him as literally 'a real life legendary samurai' and made him the main character of a Sengoku era themed game, of which there were many notable figures and is arguably most iconic era of Japanese history.
They could have used actual legends of the era like Sanada Yukimura, or Musashi or Tadakatsu, but instead they used a guy that barely existed and accomplished nothing.
They literally used him for modern, current year publicity, nothing more.
That is what people found insulting and disappointing. Obviously AC is not remotely historically accurate, but to have their first game set in Japan and you don't get to play as anyone remotely representing the era is laughable.
Your play choices are either a Yasuke, who again they literally falsely marketed as a 'real life legendary samurai' or a fictional female ninja created by same lame ass French folks. Ass. Should have just gone the full on fictional character route and give us made up male and female characters like the Odyssey or Valhalla.
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Oh, and before the false equivalency of NiOh 1/William comes up? Let's compare the two.
-NiOh 1 William was based on William Adam's, a real life foreign sailor who DID officially get the samurai title from the shogunate and was even renamed Anjin Miura. Yes, meaning he was even given a last name and that became his lineage and house. He even married a Japanese woman and had kids and lived the rest of his days there.
-The first game was also loosely based on a incomplete Akira Kurosawa movie script about a blonde haired blue eyed samurai.
So NiOh's William has several points of historical significance, including the Akira Kurosawa inspiration point.
Yasuke on the other hand was a literal nobody. He was in Japan for barely over a year, he didn't participate in any of Nobunaga's campaigns, he wasnt given a last name or an estate as a samurai would have been. And in the end he was sold back.
To call Yasuke a legendary real life samurai is laughable. To even call him a foot note is a bit too generous too.
His presence had zero effect on history and thus there was only a few lines recorded about him.
He is more like an interesting factoid nestled *within* a footnote about Jesuit priests, with said footnote belonging to Nobunaga's history.