I have found that if you keep your unit sizes relatively low, (1500 for my recent playthrough) Legendary is a lot more manageable. I have completed Gettysburg (Day 1, natch) and the only truly difficult missions were Stones River, Rio Hill and thrice damned Cross Keys. Played through under Iron Man rules only replaying Chancellorsville after the Hotfix for the Day 1 Victory.
I'm not saying any of the missions were Truly easy and Antietam was a brutal and vicious scramble for most of the battle, but it is most certainly not impossible.
Manpower isn't even a problem (none bought with Rep) but I was constantly low on Money from keeping my small brigades vetted up.