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.
After the fiasco with Mobile Suit Gundam Seed Destiny, Cagalli Yula Athla returns to piloting in K. Banpresto decided to make her first mission one that's simply plain awesome on principle: taking on the Festum in an outdated, underpowered Strike Rouge becomes a lot more badass when you realize those things can alter gravity, create black holes at will, read your mind, and pretty much EVERY canon pilot who had a chance against them had been genetically modified. If there was any enemy for Cagalli to take a swing at to show she hadn't lost anything, this was the one.
Game Breaker: Interestingly, you get a LOT of them, such as the SEED Destiny Gundams, Dann of Thursday, both Jeegs, King Gainer, Gaiking, etc. Unfortunately, given the game's difficulty, this still means most levels are uphill battles.
Parts-wise, the Orgone Cloud, which grants a level 3 barrier, double-image ability, ignore terrain penalties and a level 1 energy recovery. You can only get one by either inserting a Super Robot Wars Judgment cartridge or through the shop on your 5th playthrough, and boy, is it worth the hassle of getting it.
The Multicombo system in K is a Game Breaker in general. With the combo skill at level 3, it lets any unit attack up to six enemies at once while bypass enemy support defense entirely. They are essentially short range MAP attacks, except without all the usual drawbacks of a MAP weapon like low ammo or reduced experience gain or allies not getting any will gains from your kills, so you are free to spam them to your heart's content right from the first turn.
Rescued from the Scrappy Heap (SEED Destiny: not only is Kira Yamato more reasonable (and doesn't attack your team on sight, but agrees to join them) and Cagalli's Chickification completely nullified, but the game goes out of its way to make YuunaRomaSeinan a bit less of a coward and more caring towards Cagalli, to the point where he acknowledges his faults and steps aside when Cagalli forcibly takes command of Orb. Yuuna actually manages to LIVE through everything, on top of this.)
The same can't really be said for the ZAFT characters, though (with the exception of Athrun Zala and maybe Heine Westenfluss).
The Scrappy: Mist Rex is, beyond the shadow of a doubt, THE most hated Super Robot Wars protagonist ever (and the second-most hated character period, just after LeeLinjun (or third, after Wilhelm vonJuergen). He spends most of the game complaining about how much better than Earth his homeworld is, and he acts like an asshole to cast over their character flaws (berating Go for his women troubles, telling Van to get over his bride-to-be's death, insulting Kira's idealistic pacifism, and denigrating inexperienced pilots like Gainer, Ryuji, and Daiya). The ultimate expression of this is his reaction to the ending as a fun adventure, completely glossing over Soushi's death who, mind you, sacrificed himself to ultimately destroy the Eldritch Abomination that was RESPONSIBLE FOR MIST'S HOME WORLD GETTING DESTROYED. Made worse by the fact that a lot of his attitude is hypocritical, such as one ending putting him in a harem situation after insulting Go for the very same reasons, or some instances where he recklessly charges at Edax forces and gets his ass handed to him...after berating other people for doing just that. Japan has a fandom meme involving photoshoping Mist and his mecha into announced SRWs to screw with other fans.
He is also seen bullying some other characters in fanart, such as Shiony
Mist Rex might actually be the most hated character in SRW now that Lee Linjun had a Heroic Sacrifice at the end of the 2nd Original Generation to save Ing Egret. While it is debatable to say yet that Lee Linjun has been Rescued from the Scrappy Heap, the fact that he has done some good has elevated his position in the eyes of gamers and has made him not as hated.
This and various other reasons have fans seeing the whole game as this compared to W and L.
Scrappy Mechanic: Many players consider the Partner Battle System to be useless, especially later on in the game when many standard mooks have access to the Combo Attack. Partnering two units also prevents either from using Combo Attacks, further discouraging its use.
Those who do want to try the Partner Battle System will find it does have some uses, such as always having mecha in range of each other to combine or use Combination Attacks. It's a question of weighing up the benefits of these vs. being able to use a Combo Attack.
Scenario 5 pits you against EVERY SINGLE NAMED BADDIE in the first half of Overman King Gainer, INCLUDING Brunhilde, that in-series attacked friends and foes alike, yet here only attacks you. Fortunately, you only need to destroy Brunhilde...except it has 10000+ HP (at Scenario 5, no less), and you've got around 7-8 Fragile Speedster bosses attacking you at the same time. Fortunately, the secnario has easy Mooks to handle.
Scenario 6 kicks off the GUN×SWORD plot, only Van is being attacked by a lot of Darius EmpireMooks. You basically have to survive for two turns until the Daiku Maryu shows up to aid him, which is easier said than done.
Scenario 7 is a mix of the two before: it begins with the Dann of Thursday, plus El Dora V (that unfortunately is an EN guzzler), Brownie and Mist being attacked by a bunch of Overman King Gainer grunts and bosses (again). You'll have to endure a few turns until the other heroes show up, only the enemies also get two tough bosses as reinforcements: Cynthia Lane and Ray. Like Brunhilde in scenario 5, neither of them were with the Siberian Railroad, but fighting for themselves; however, they only attack you and you alone.
Any Virtual-ON-based scenarios: the first two are long and boring, set in corridors against mooks with MAP Weapons, while the last one is on an open field with more grunts like that AND Elite Mooks with huge HP supplies AND the Attack Combo skill that allows them to crush your support units.
Scratch that: let's just say that any long corridor-based scenario is long and dull (we're looking at you, final Fafner scenario).
In one Mobile Suit Gundam SEED C.E. 73: Stargazer scenario, you must prevent any enemy unit from reaching a certain point...IN SPACE. This sort of mission objective isn't a problem, except in a game where many series and their respective mecha don't go into space, they're stuck with a relatively low B-ranking for terrain rating (and terrain matters A LOT). To emphasize, a Lightning Bruiser such as King Gainer in K can easily get hit and often miss in space. Ouch.
They Wasted A Perfectly Good Appearance: Despite the proper SRW appearance of Godannar (the series made its debut in Scramble Comannder 2), units like the Goddinner, Genesister and Volspinner do not make an appearance, let alone even be mentioned in K.
Their pilots do make appearances in some cutscenes though, just not their machines.
Koutetsu Jeeg's various frames. One of them had drill legs!
Despite the changes to its storyline, SEED Destiny still got the short end of the stick in several areas. So what if the Minerva's Captain Talia Gladys dies in the game? Let the players use it instead of the Archangel, for a change.
Also, Sky Buster Gaiking doesn't appear, despite the individual forms appearing as Combination Attacks.
Similarly, Triple Gaiking (the combination of Gaiking and its three support robots) also is unusable.
K marks the first time the actual Virtual ON MARZ storyline debuts (since the cast treated their Super Robot Wars Alpha 3 appearance as a kind of "bonus level" from their world), yet you only get to control 4 of the 9 MARZ characters that are heroes and/or help the player on certain stages.