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.
Game Breaker: Doom mode in XOP Black. You can only hold one Reflect Shield at a time, but it turns bullets into DAL chips and every time you collect a set number of DAL chips, you gain a new one if you don't have one yet. Shooting enemies also grants you DAL chips, even if you're only damaging them rather than destroying them. When larger bullet patterns show up, especially in boss battles, it's very possible to scoop up enough DAL to recharge your shield before your current shield even ends. It's very possible to cheese bosses, even those in the True Final BossRush, through a nonstop cycle of shield -> collect DAL -> new shield -> repeat.
Nightmare Fuel: Many of the enemies in stages 3 and 6 of the original, especially stage 3's miniboss: a giant mutated brain.
In XOP Black, stage 2 (Skies) on Cancel mode. In Cancel mode, enemies will turn their projectiles into DAL chips upon death...unless they haven't fired any bullets, in which case, they explode into fast orange suicide bullets instead. Skies in particular has a constant fleet of drones that home in on your ship and never fire—harmless in other modes, but very much Demonic Spiders here. Because of them, scoring well on the Skies stage is practically near-impossible.
In earlier versions of XOP Black, unlocking Insane difficulty. It required you to 1cc the game on Very Hard. To those who don't know Shoot 'em Up slang, it means clearing the game on Very Hard without continuing.