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.