These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Air Fortress
Demonic Spiders: The evil spacemen, no doubt. This is because A) They have something approaching a survival instinct when it comes to dodging your shots, but more importantly B) They're the only enemies in the game that regenerate after being killed.
Goddamned Bats: Most of the enemies in the Fortresses, to be frank. Most of them aren't that dangerous individually, but their tendency to hang out in groups, combined with your very short damage invincibility, makes it easy for you to take gobs of damage with little way to avoid it.
Good Bad Bugs: Enemies in the Air Fortress, especially turrets, occasionally "spawn" pre-destroyed. Given that, with a few exceptions, destroyed enemies in the game do not regenerate, this is pretty helpful.