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: There's a list of 'Power Weapons' on the game's wiki, though few are outright Game Breaker guns in practice.
The M590 CQB and its variants can deal 95+ damage at the same range a Remington 870 would deal 80. The regular M590 itself once had this power, before being nerfed. The CQB version, thankfully, is a (somewhat) rare Supply Case weapon.
The regular M590 CQB is actually relatively easy to accquire these days, coming from the MYST-G case, which is purchaseable for the cost of 4000 GP.
Specialists, characters with impressive stat boosts and exclusive access to extremely powerful weapons, including flamethrowers, 6-shot grenade launchers, chainguns, and airstrikes.
Of course, buying with NX nets you more powerful gun variants, as well as removing the rank restrictions.
Honestly, if you add up all the weapons people think are Gamebreakers, it'll be something like over half of the weapons in the game.
They Just Didn't Care: Many forum goers and actual players share this feeling in regards to the developers and Nexon. Though it's not so much a question of if they care, as what they care about. For a start, there are more NX guns than GP guns and a popular hack seems to be the inspiration for a cash item.