Don't disagree.
A few thoughts come to my mind:
Discipline was much harder back then. No Hollywood-like discussions between say privates and captains.
Feeding an army is always a problem but I think it got better in the 20th century than before (preserved food etc).
Medical advance was already mentioned. Scharnhorst died of an infection of a bajonet wound...
You were in very close physical contact to the enemy. Imagine standing upright with your musket in roundshot fire, sometimes for very long. Or trading musket salvoes with other dudes 50m from you. Always the thought"will I make it for another second, how will I be maimed or killed?".
My grandfather served on the eastern front in ww2 and always told it was very comforting to feel mother earth and to dig as deep as you could even though a full hit could still occur any time. It was not "unmanly" to hit the ground any more.
One thing that was maybe better was the killing zone was not too deep. Run a kilometer from the enemy and he can hardly reach you. Sit at your tent at night with a cosy fire, not a problem if not in the front line. Some of the 18th century armies even went to winter quarters and were doing more manoevering than fighting because they were very costly to drill and you don't want to ruin your precious army. No fighter bombers when marching, no IR sights, no poison gas as in ww1, no tanks no aircraft, no mines, no missiles.
I'm glad I served in the forces in the 1980s, not in the 1780s.