The United States Army had developed reasonably practical uniforms for the Army prior to the Civil War. The truth of this can be seen in that no major changes in the uniforms occurred during the civil War. The sack coat and forage caps were very practical and even the frock coat and hat were not entirely unpractical. The Army overcoats were warm and functional. I am less sure of the mounted jacket. Post-Civil War uniforms were similar to the Civil War uniforms and it is noteworthy that the mounted jacket was discontinued after the war. I do have to wonder what the Army was thinking when it adopted the spiked helmets in the post-Civil War years. Because I have never worn the spiked helmet perhaps I am mistaken on how practical it really was. Do we have any Indian War re-enactors that have worn the spiked helmet?
Good question and I've wondered about that. Thank you for bringing this up! The CW-era uniforms, especially the sack coat, seem the pinnacle of comfort and practicality to me, compared to earlier ones (not as familiar with later ones), and they echo a civilian emphasis on looseness and practicality coming about in the 1850s too as the looser-cut sack coat replaced the tight-sleeved paletots, and even the frock coat was more loosely cut in the sleeves than the tighter frock and tail coats of earlier. Same for more loosely fitting trousers. The only thing I'd claim was the worst for practicality was the kepi or forage cap. Yuck. Sunburned ears are the least of one's worries. But add a wide-brimmed slouch hat like western troops did, and you're good to go in all weather.
Was it just luck that the CW happened to occur at a time that fashion was giving away to practicality? Or were the before and later uniforms really just as comfortable in their day and the difference was only an illusion? I've wondered how War of 1812 reenactors feel about their uniforms--for land use, comparing apples to apples, since I'm not sure if things might be different on shipboard. But the War of 1812 uniform has always looked particular uncomfortable to me. And even Mexican war reenactors--how do those uniforms compare?
There's also the influence of climate. In the humid, cold-then-hot east, wool had to be the fiber of choice. Did it differ out west or in Mexico? Were there problems where a uniform was designed for one climate, then worn to a war in another?
Again, thanks for asking, and I'll quit rambling and read with curiosity the answers from people who know about this sort of thing.