One glaring thing i noticed that people have missed is that Yasuke had no last name (family name). In ancient times, Japanese peasants didn't have last names only the nobility and the samurai did. So the fact that Yasuke has no last name/family name and was never given one means he was never accepted as a samurai. (besides the fact that you know....the title of samurai was passed down through blood and you couldn't just become one anyways)
Of course this is just if Yasuke did exist, but he didn't.
One qualm I would have about it is that it doesn't prove that the so-called "Yasuke" learned Japanese in 3 months. It only claims that within 3 months of meeting Nobunaga, "Yasuke" would have convinced him of his skills and loyalty, and would have been welcomed as one of his samurai. "Yasuke" could have learned Japanese before the meeting every happened, which could theoretically have been over a period of decades, as incredibly unlikely as that could be. Languages are learned much easier when you're immersed in the culture and surrounded by people who speak it, giving you no choice but to learn it to integrate yourself. But who knows; if the man's name was really "Yasuke", then he could have been raised from infanthood by Japanese immigrants, before ever arriving in the country.
From another post, apparently, there was only one year between "Yasuke's" supposed arrival to Japan and Nobunaga's death, and no major war to speak of during that time that would have required him to find more samurai to bolster his forces. If that second post is accurate, then the length of time "Yasuke" would have had to learn Japanese within Japan would be closer to 9 months, before meeting with Nobunaga. Still nowhere near believable, and I still don't believe that he actually existed, but at least it doesn't make an assumption not supported by the premise.