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: The Shadow robot. If you stun your opponent, grab them with a shadow double and perform a combo on them, the enemy will only get up stunned again, allowing you to repeat this ad infinitum. Effectively, so long as you can stun your opponent once and perform this trick, you win.
A similar trick could be performed with the Chronos robot if you froze your opponent in the air, combo them and froze them again rendering them absolutely helpless. Even the CPU on Ultimate difficulty could be taken down easily using this method. The Chronos has a combo that involves using its time freeze projectile, attacking the opponent while their frozen and knocking them into the air, so you could time freeze them again, and knock them in the air while they were helpless. If timed right, this combo is pretty much unstoppable and makes even the Ultimate Mode seem like a joke.
The Gargoyle mech. It has a nigh-unblockable aerial throw and decently powerful attacks, and a shoryuken with the most ridiculous invincibility frames ever. To make matters worse, it can chain the air throw out of the shoryuken. Oh, did I mention that everyone online plays on Hyper Mode which makes the game moreStreet Fighter, giving rise to the impossibly powerful 'Triangle Jump' technique? Ironically, Gargoyle's countpick is Jaguar, which is still fairly decent (it's the starting mech in tournament mode).
See also the Katana. Not gamebreaking unless it's playing with all upgrades—in which case it has the stupidly powerful Fireball projectile and a five-hit Shadow Rising Blade rush which it is completely invincible for the duration of.
We also have the Shredder, who is above average at best until you click over to Hyper Mode. As game guru Robyrt put it, "If you land a flipkick or headbutt with Shredder, your opponent should die".
Also, some of the possible combos (Robyrt has some cool ones on his site) are flatly ridiculous. Against a suitably slow opponent with a suitably fast pilot, Jaguar can chain two Overhead Throws (air throws that hurl the opponent across the screen, in a game without dashing) together. And the Nova can use its aerial bellyflop, which knocks down an opponent, and chain it into the grenade...then hit the opponent as they get up, knocking them back into the grenade. Similarly to Jaguar, it's possible for Flail to chain its Spinning Throw into itself, but that is even MORE ludicrous, as it sends them flying almost on the ground.
Using Steffan or Jean-Paul as your pilot will make matches much easier considering their nicely balanced stats, and their ability to function with most of the robots pretty well overall.
Gargoyle is technically the best robot, but it requires you to have consistent execution. Electra, on the other hand, has stupid good punch attacks, one of which can lead to an infinite if your pilot is fast enough.