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: Maya. It's obvious why she needs to be unlocked.
First, there's her spear - an energy projectile that counts as a melee attack and travels through walls. Many bosses, but especially Ikarius (who otherwise has That One Boss qualities) become downright sad with it.
Even worse is perhaps the wall. An energy barrier that not only damages enemies that come into contact with it, but also reflects bullets which can damage the attacker. The more bullets there are, the more effective it gets. And the further you get in the game, the more bullets the boss will shoot at you. See where this is going?
Seeing how in the comics her main job in the army was to be a gaint 5k flying battleship's main gun the devs got her right.