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: In Assault, the Rafale and EFA enemy planes. The Rafale is the enemy version of your FS-X, meaning it has the Ninja Beam and excellent dogfighting ability. It can give you a headache if it chooses to engage you, or you do so and lose sight of it. The EFA has Phoenix missiles, meaning it can be quite dangerous to your team as a whole if it gets more than one in its sights, as well as being as maneuverable as the Rafale.
Game Breaker: The Ninja Beam in Assault. It does insane amounts of damage, in a Boss Game where one of these will leave most bosses half-dead, or damn near One-Hit Kill it. The otherGame Breaker in the game is Spanky's plane's main gun, the "Beam Sabre", which is essentially a weaker Ninja Beam with infinite ammo and shorter range.¨
Any of the dialogue, from any of the games, in fact. Especially the second game.
That One Boss: In Assault, the Spriggan and the Fortress, both being Damage Sponge Bosses as well as Mighty Glaciers. The Spriggan is made hard primarily because of its accompanying enemy aircraft which will annoy you all throught the level, as well as the time ticking by and how you'll be hit hard and mercilessly by its weapons if you aren't smart about it.
The Fortress is extremely resilient, and protected by a ton of Anti-Air emplacements, making a head-on attack suicidal. It also has enemy aircraft around, like always. Even if/when you take everything out but the boss, remember the Fortress itself is target-less once its adjacent SAM sites are gone. In other words, you have to manually aim your weapons at it, and at a specific area to boot!. This makes your missiles (the best and usually your main way to kill enemies) virtually useless unless you're using Glenda, whose Rockets don't lock on, and therefore, always shoot out the full load.
That One Level: In Assault, the Air Battle and Fortress levels. Air Battle is annoying because the best way to finish it quickly is to damage the boss as much as possible before it fully spreads its wings, as it won't fire back until it does. The problem is that there's lots of B-22s shooting at anything above their altitude level, and several enemy aircraft trying to kill your team, with there being at least 2 F-22As. If you ignore them, you'll find yourself eating a lot of bullets from one tailing you, and your team can easily be picked off without your support. If you go for them and don't down them quickly, you'll find yourself hard pressed for time to kill the boss before it destroys New York.
Fortress, meanwhile, gives you 15 minutes to destroy the Fortress, minutes you may find not sufficient if you don't work quickly and effectively. Going straight for the boss is suicide, since there's a TON of SAM sites around it that will shred you to pieces quickly if you don't pick them off one by one outside their targeting range. There's, of course, aircraft around to delay you. And when you DO destroy every SAM site around the boss, you'll find there's no target to lock on to—The boss is right there alright, but you can't lock onto it, meaning missile types where the damage done is reliant on lock-on are made virtually useless (for reference, most of your characters suffer from this problem; Glenda doesn't because her Rockets don't lock on, and Volk because his is just one missile, locked-on or not). To make matters worse, although the Fortress won't shoot back once it has no more SAM sites, it is very, very, VERY resistant to damage, meaning you may find yourself pressed for time if you don't do everything up to this point effectively. And if you do run out of time, no worries, you don't get a Game Over...The Fortress will simply shoot at you.With Ninja Beams.Repeatedly. The sad thing is, it can easily kill you with just one.