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.
Touhou fans may find the last one familiar—the first part of Suwako's theme is strongly based on it.
Demonic Spiders: Anything with missiles, especially because missiles in later levels have very good homing capability and will hit you more often than not. Compounded when there are background obstructions in the way.
Jet planes are the worst of the lot. They can fly, are hard to hit, and you have very little time to shoot them down before they fire a missile. The best method to deal with them is to stay high, then drop low when they fire their missile- if the missile goes offscreen, it disappears.