I know it's a bit vague, but I'd say backgrounds, specifically for realistic locations, I don't mind simple shape backgrounds.
You have to draw things like plants, buildings, road signs, posters, tables, bystanders, etc. Then you have to get the angle and scale right so the characters don't look awkward. In a comic scene you'll have to double check everything's consistent to avoid disappearing/reappearing objects. It's even harder when done traditionally since you don't have layers to fall back on.
It's so much work, but it's necessary to make the story's world feel lived in. Sometimes I get so frustrated that I feel like cheating with premade manga assets, but it wouldn't match my art style and I'd feel guilty.