Well, it's about time I chimed in again with my thoughts.
How we rate someone is often done by comparison. But, it's not the only way. Thanks for the reminder
@7th Mississippi Infantry .
I think Ulysses S. Grant is underrated in terms of his humility. As
@diane pointed out early on, Sam Grant came from humble beginnings. There was no sign of the greatness that would attach itself to him eventually and, when it did, he never seemed to let that go to his head. I think his mother had a lot to do with that. His father, on the other hand, was proud of his achievements and wanted others to see what he saw in his son. Many thought Jesse Grant 'overrated' his son, and Ulysses paid a price for his father's 'meddlings'. To my mind, that just further ingrained his sense of humility ... born out of the experience of humiliation. When he thought to be proud, as in when he first came home in uniform from West Point, he was brought low very quickly. It taught him a salutory lesson. Even if an undeserved one.
He appears to be underrated as the man who won the war for the Union. Lincoln finally found someone he felt he could work with and Grant obliged. Where the label of 'butcher' is applied there is a need to remember who applied it first. So, in as much as we can 'number crunch' all day on casualties, the fact is taking the offensive in a war is often going to engender more casualties. To 'go on the offensive' means to take the initiative by beginning to attack or act aggressively. Grant had some cause for regret, but he knew no other way to win the war than to keep moving forward. After Fort Donelson he wanted to move forward more rapidly with the sense that it would save lives. And bringing the war to a close more swiftly was always going to do that. He also had the unenviable task of moving from the Western to the Eastern theatre of the war and having to face Robert E. Lee, the consummate professional soldier. His men were in awe of Lee. Somehow he had to push through that psychological barrier, and he did. So, rather than compare him with Lee, I would say he is underrated in terms of how he managed to defeat Lee in spite of the odds that Lee presented.
There are those who would underrate him both as a man and a soldier. He had a chequered history after leaving the army, and a reputation that came from his decision to resign which stayed with him throughout his life. It became a means to undermine him, and in many people's minds is still another way to 'underrate' Grant. Just look at his reputation. He was a drinker. He couldn't handle his liquor. A man with a drinking problem cannot be rated highly. And so those who believe this continue to underrate him.
As a family man he struggled to make a go of things after his initial departure from the army. I think Julia upheld her end of the bargain here by never giving up on him and always supporting him. She never underrated him as a husband and father. So, I will take my lead from her.
Then we have Grant, the President. Not just for one term, but for two. I don't think there's any danger of Grant being 'overrated' as President since most people agree there were downfalls in his Presidency. But, is he underrated? Maybe. And that is in terms of what he tried/hoped to achieve at times, but ulimately failed to achieve because he was a member of a government where factions existed.
So, all in all, I think Grant is 'underrated' in general. And that is without comparing him to anybody.