Bullet hells are slower-paced and give a lot more warning (bullet travel time, mob wind-up time) before a particular threat can kill you. It's rarely the end of the world if you forget that an enemy spawns somewhere, but it can force you to dodge an ugly or awkward wall.
Contrast a shooter like R-Type that relies on speed, tight spaces and enemies going into blind spots. There's a lot of "That's bullshit!" moments and I've generally preferred bullet hells for that reason.