I can't speak for StoryMode. I found Normal to be alright, with couple difficulty spikes here and there. Act2 can be frustrating but that's mostly because you can easily run into content that outlevels you and you need to figure out in what order you are supposed to do it.
Personally, once I understood how equipment works, I found the game managable. My initial mistake was not to upgrate gear after level ups, as itimization in D:OS is tedious and boring as hell.
