It was the lack of communications that Generals could not get almost any accurate information and how artillery could be co-ordinates. Without artillery co-ordination you were very very vulnerable. It gave the defender great advanategs as his information was normally better, if he knows how far teh enemy have advanced, dumping artillery fire on his former trenches is often really easy.
It is not the scale. It is the issue of having encountered problems (assailing entrenched/hidden enemies) without really coming up with a solution.
No the type of warfare was so different as to make any parallels mostly meaningless. The US Civil is a lot close to the Napoleonic period than WW1. Napoleon could have recognized the civil war, he could not have ww1. Minnie Ball muskets were an advance but compare to magazine rifles it's not close.