I have yet to read his memoirs (I will get one of the annotated editions when I do), but my understanding he says little about his presidency. Understandable, considering his health and what would have the greatest interest among readers (he was writing the memoirs in part to secure money for his family after his death, after all).
Considering Jefferson Davis managed to redeem his reputation by writing a memoir of his time as Confederate president one wonders how Grant's presidency would be viewed differently had he had his say on the subject.
It would be great if Brooks Simpson would someday finish his biography of Grant's presidency, but it's been 24 years since the publication of Triumph Over Adversity so I'm not holding out hope. For all I know he might have abandoned the project.
Even the most honest memoirs will give more attention and focus to what the author thinks is important and less on what the author considers less important. But the author's focus can be as revealing as what they have to say.
I know of one non-military autobiography where the author omitted any mention of his relationships and children, and removed other material during the editing process for the sake of making a book that read better. (And it was certainly a very a good read.)
On a different part of the spectrum, I know of a retired public school official who self-published an autobiographical work that is a staggering 700 pages, which is not suggestive of good editorial restraint. Because many of the people in the book are still alive he made the unusual choice to fictionalize the names of just about everyone, including himself, and even combine some people. It leaves serious historians like me who might want to wade through the book unsure of how much he wrote is truth and how much is embellished.