Another glaring weakness of Ulysses S. Grant was that he had a serious drinking problem. When he was having issues in his marriage (which depressed him, as a devoted husband) or when there was little happening on the battlefield to occupy his mind, Grant had a reputation for going on serious benders.
While he was in command of the siege of Vicksburg in 1863, there was a six month lull where the troops, for the most part, didn't move. Grant, on one occasion, became so blindingly drunk that his officers wouldn't let anyone see him.
Word of his escapades reached Lincoln at the White House, as some in the Army and Cabinet suggested he be sacked. Lincoln replied "I can't spare this man. He fights." His command tactics and resolute persistence on the battlefield, I would argue, more than made up for his problem. Nevertheless, it was a weakness and he would have been a better commander had he been consistently sober.
One of the major points of criticism of Grant came after his campaign against Lee at Cold Harbor. In that particular battle Grant seemed to lack real concern about the casualties that were a certainty in a frontal assault on the basically impregnable fortress the Confederate army occupied. He still ordered a frontal assault and the bloodiest few minutes of the Civil War followed. Thousands of Union soldiers died in completely hopeless attacks against the entrenched Confederates. And this was just one example of the times that Grant seemed willing to press for "unconditional surrender" rather than making more reasonable decisions that would lead to victories that weren't as costly in terms of lives.