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.
Demonic Spiders: Some of the 'mooks' have a habit of flying just so close to your plane that you'll have to drastically slow down just to avoid overshooting (flying past), or will fly a tight loop to keep throwing off your own attempts to generate angles for missile shots or gunfire. Particularly annoying when it's in a skirmish, as decelerating to keep them in your line of fire will leave you vulnerable to other enemies' attacks, and deadly when (not if) the enemy aces do it to you.
The enemy aces in general. They always show up at the end of a mission, which if you have been spending it shooting down enemy planes may mean you are running low on ammunition. They will almost always outnumber you (with the exception of the Gelb Squadron) and are exceptionally coordinated. As mentioned above, they fly in tight loops and while you are chasing them they often have someone else chasing you at the same time. At the higher difficulties, staying on top of them requires pinpoint accuracy to avoid blowing all of your ammo on nothing but the sky and co-ordination with your wingman to keep them from getting the jump on you. You don't get the latter on the Ace of AcesBonus Level.