There was a good discussion of Grant here, led by u/Wulfrinnan

Some of the bad things said about Grant can be linked to the Lost Cause school, which tried to portray his administration and Reconstruction generally as corrupt, and so the Southern resistance to it as therefore noble. Grant was not corrupt, but did have problems with corruption within his cabinet and his family. A military man, he was loyal to his staff and family and did not comprehend how one of his staff ( or his family) could be disloyal to him , be tempted by greed and their proximity to power. He learned, however.

He has also been faulted for giving up on some very good things he began to do- better relations with the Native nations, and on implementing and enforcing Reconstruction. Ron Chernow's recent biography has tried to make that the fault of the Republican leadership and the avaricious US population, greedy for more Indian land and uncaring of civil rights for freedmen. There's a lot of truth in that, but I think Chernow has taken his typically kindly approach with one of his biographical subjects a little too far. The earlier standard biography, by William S McFeeley, is less forgiving. McFeeley points out that Grant became quite quickly a bureaucrat, with no ideals or purpose except to come into the office. When he considered another campaign for President, in 1880, he couldn't actually say what he wanted to do; he just wanted his old job back. I think this is perhaps the saddest thing about Grant: he was the most popular man in the United States in 1868, and continued to be popular, yet instead of using that position and campaigning fiercely for his goals he shrugged and walked away from them. It was a moment in which he could have become truly great, rising to the occasion. And he could have failed, despite that. But he gave up. And he knew this: his Memoirs notably say almost nothing about his time in office, relegating it to a few rambling paragraphs at the end (on Reconstruction, he makes some vague suggestion about helping the freedmen by sending them all to what's now the Dominican Republic). He had been a rather workaday General, calm, humble, persistent and not prone to grand speeches, and he became a rather workaday President, with no grand vision; content to keep his office hours and do what he was told by his political advisers. In normal , peaceable times, that would have been fine. But his were not normal, peaceable times.