There's been a lot of talk about Marco Polo in this sub but not this question.
Reading his book there are things that are obviously true, who ruled where, who won which battles, what people sell and produce in different cities, the customs and superstitious may be exaggerated or misunderstood but one can trust them.
Then there are things which are obviously not true, like everything he says about Prester John, this are things that he probably added to make the book more enticing I guess.
Then there are things that aren't true but Marco didn't know, for example he mentions that Mogadishu is in an island but it is in the coast of Somalia, or the relationship between Nayan and Kublai. I give him a pass with those, information was very hard to verify in the 1300 and he was telling all this from memory.
The problem are the things that are feasible enough to be true, or exaggerations of truths, but that is hard to be certain.
For example, he mentions that in Hangzhou there were no beggars, because homeless people were given houses and food, was that true?, is there some truth to it?, another example is he mentions cannibalism in Japan of war prisoners, if that happened it probably was due to the Mongols destroying crops and food supplies, and probably didn't happen again, but it could have happened.
And there are a thousand more things like that in the book which I am skeptical of and don't know if I should trust or not.
Igor de Rachewiltz, perhaps the top Mongolist of the last few decades (not anymore though, he just died last year unfortunately) in his "Marco Polo went to China," in response to Frances Wood's "Did Marco Polo go to China?" has pretty definitively shown that Mr. Polo was in China, and that there is an excellent amount of information, sometimes revealing, about Yuan China. The modern scholarly consensus is that he is actually quite reliable.
The problem with Polo though, is that we don't have the original manuscript. There was so many translations and additions over the years that it becomes problematic to find out what was actually in the original, or what Polo himself may have decided to include, not to mention what other writers added or removed. For this reason modern scholars, if working with Polo's work, will use several manuscripts rather than relying on one translation. There is one which we hypothesize to be fairly close to the original, but for the life of me I can't find the name of it right now.
The other issue is that Polo himself didn't write it: it was transcribed by his cell mate Rustichello, whose intent was to make a good story. So it becomes questionable whether certain elements were played up by Rustichello, or Polo himself, to make it appear that he traveled more than he did (particularly on his return voyage), or was more important than he actually was (such as suggesting that he was a governor of a city for a while, which seems unlikely).
In the Middle Ages Polo doesn't seem to have been believed all that much, which has impacted the reputation of his work, which is a shame because it appears the tendency to literary exaggeration has disguised what is a very useful, if at times cursory, work.
For more on the matter, I would recammend John Man's "Marco Polo: the Journey that Changed the World." While not a Mongolist, man goes extensively into arguments around Polo, travels much of the route and shows conclusively that Polo's journey is largely true. As it was written for a larger audience, it is very accessible for people who don't have much background in the area.
As well, here's a copy of Rachewiltz' "Marco Polo went to China." http://altaica.ru/LIBRARY/rachewiltz/Rachewiltz_Marco%20Polo%20went%20to%20China%201997.pdf