Both of which were extraordinarily risky maneuvers. The difference is that the southern generals knew their own men, what they were capable of, and also understood the minds of their opponents.
If the Union had had better generals, or better scouting, both battles would have been a disaster for the CSA.
If you divide your force in the face of a superior enemy and win, you are bold. Do the same thing and lose, you’re reckless. Them’s the breaks.
Custer had been bold in many battles and got away with it. This time it turned out he was reckless.
Do you know the real strength of Indian bands?
It’s not CSA, it’s not well organized or led. A chief can lead warriors, but they can also move by themselves without question. It wasn’t a military force and it was totally lacking organization, discipline and fire control.
Custer knew it. And he was right. There’s no fire control or tactic by the Indians that day - all what they did was slaughtering Custer with heavy losses (see http://www.custerwest.org - “Indian casualties”) and after having been surprised twice.
If Custer had had his full regiment, Little Bighorn would have been another Custer’s victory.