I was annoyed by the spammy block damage from festive creepers and extreme frequency of Blazing Juggernauts, so without the option to reduce spawn rates, I disabled them. Now the nether is very empty, with very few mobs spawning. My guess is disabling a mob doesn't actually remove them from the pool, it just stops them from spawning when the game rolls them. I spent several minutes trying to find even a single ghast for a recipe that requires a ghast tear, and zombie pigmen aren't very common either: in vanilla I would have been swarmed with both of them.