At higher difficulty the game it's only making the enemy more tanky and therfore the overall progress is drastically slow down, you need for full revolver load to just kill basics enemies.
What I expected is that enemies make more damege, are faster and in higher number, but I don't like the feeling that each small enemy is just a boss that takes minues to clean up.
I feel this is a problem with the difficulty spikes between each difficulty setting, which spikes up too much but which might also be expecting co-op groups to handle those difficulties.