I think that there are some ways in which it is harder to be a leader today and some ways in which it is easier.
The major way in which it is easier today comes about because of communications. During the Civil War, unit leaders might be completely out of touch with their superiors. Those trying to command would be out of touch with their subordinates. At the best of times, they would have to rely on couriers carrying messages to and from the units. There was no kind of real time communication that would allow commanders to understand the situation in a battle and to get information to the people who needed it. This made military leadership in those days very difficult.
Perhaps the most important way in which it is harder today is that military objectives are harder to achieve today. Today, the US is typically fighting in places like Iraq or Afghanistan where the mission is as much political as it is military. Leaders are not just expected to be able to fight. Instead, they have to be social workers and diplomats as well as fighters. This is much more complicated and difficult than things were 150 years ago.