I'm watching it at the moment, but I can tell you that the magic ninja-arab assassins did not exist or try to kill Kublai. As the show states, Hulagu wiped out the Hashishshin a generation before the events of the show. Marco Polo did write about them though, and they did try to assassinate Mongke Khan before they were destroyed.

Kököchin being a peasant imposter is the result of creative license as well. Oh, and Ariq Boke's rebellion was a much bigger deal in real life; it began immediately after Mongke's death and lasted 4 years. It was a pretty bloody civil war that was definitely not settled mano e mano. Ariq surrendered after losing all his allies through attrition, and died in prison two years later. If Kublai did kill him, it was with poison. Also, Hulagu was the brother of Kublai and Ariq, not their uncle. That was a weird random mistake.

Not to interfere with the interesting posts already made, but here are some previous posts:

How historically accurate is the new Marco Polo series by Netflix? - mostly compares the show to Marco Polo's The Travels.

• In the Netflix series "Marco Polo," Marco Polo, a Venetian, has a relatively easy time communicating with people in the Mongol court. Would actual western traders be able to communicate effectively without translators? If so, how? and In Netflix's Marco Polo they have a character befriend Marco who speaks "Latin"(English). How likely would this have been for Kublai Khan to have people who could translate readily available? - both consider how the language barrier is portrayed in the show.

Once upon a time, I studied Ugaritic and really enjoyed learning its cuneiform script. So, when I saw these texts in episodes 2, 5 and 6, I was pretty excited to see a cuneiform script used in a popular show (Sorry for the poor quality. They're just photos I took of the TV screen with my phone). It turns out that they are written with Old Persian cuneiform characters. This script went out of use around 300 BCE. In fact,

The last known cuneiform inscription, an astronomical text, was written in 75 C.E.

a quote from Wikipedia citing Marckham Geller, "The Last Wedge," Zeitschrift für Assyriologie und vorderasitische Archäologie 86 (1997): 43–95.

On the one hand, this is a pretty trivial matter. On the other hand, rather than use one of the many contemporary scripts used in Eurasia, someone decided to use a script that had been out of use for over 1000 years for some props that the camera focuses the audience's attention on (even if it is very brief). I have no idea why they chose to use it. I find it very cool, but I also feel it means the art department, set designers, etc. were allowed quite a bit of freedom in their decision making.

Given the inaccuracies that users have pointed out here and in other posts, my advice would be that if the show sparks an interest in a particular subject, then I encourage you to follow that up because that period in history can be quite fascinating. Otherwise, I'd take the show with a grain of salt. I'm sure there were plenty of things in the set design that the show did get right, but it seems there were plenty of liberties taken.

Edit: Formatting