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.
Demonic Spiders: Rancors could be this in the original game, especially on Carida, where they have a spawn point cave. They are capable of destroying infantry platoons and most vehicles (including T4-B Heavy Tanks) in one swipe, and they have a lot of health. Chances are you'll lose a lot of units before you kill just one. They can be a pain for the attacking side on Carida, as their cave is not too far from the first landing zone, and the AI for unaligned indigenous units was incredibly aggressive in the original game, meaning that they'll be coming after you as soon as your attack force is prepared. In Forces of Corruption, the attack power of rancors was nerfed and they weren't as actively aggressive.
Game Breaker: In the original game the Empire had a moderate advantage over the Rebellion but it wasn't until Forces of Corruption when things got out of hand. The Consortium has superior units across the board (their basic infantry can easily kill the units the game outright states they are weak against with a simple upgrade) and they can gain access to some of the best units of the other factions. If they built a palace on every world then credits were almost a non-issue. Not to mention that their space units are even worse. They have a ship that can move through asteroid fields without taking damage and also has a cloaking device, not to mention that they have weapons that ignore shields.
The Death Star II in FoC. Yes, you get to command the Death Star II. Just like in the movie, it had the ability to obliterate capital ships in a single shot, and the only way to destroy it was to use Rogue Squadron's special on the overworld map.
The original Death Star in EaW was pretty close in that it completely removed having to go through a land battle (though also removing that planet's special ability in the process)
The final mission of FoC's campaign gave the player command of the Eclipse, which was basically a Death Star II shrunk down onto a Super Star Destroyer frame. Complete with a superlaser that, unlike the Death Star II, was capable of targeting and obliterating corvette-sized ships as well as anything bigger. Even though in the campaign the Eclipse was still under construction and thus the superlaser was the only weapon available, there are game mods that give the player full access to the Eclipse in Galactic Conquest mode, complete with full armament.
Goddamn Bats: On Polus, the Pyn'Gani are armed with ion blasters that can immobilize your infantry and vehicles for a few seconds, and these blasters are their primary weapons.
The Unfavorite: The Zann Consortium appears to a meta-example for the fanbase considering that most of the popular mods to the game don't focus on it. Not that it needs them very much, however.
Villain Sue: The Zann Consortium. They are a crime syndicate powerful enough to score repeated victories over the Rebels and the Empire (that we've never heard of before nor does the game give a good explanation why), including forces led by Thrawn, and at the end they manage to raid the Emperor's personal bank accounts for the money that would allow them to become the dominant crime syndicate in the galaxy, to the point that only Zann's personal preferences stop him from being the next Emperor. Such an overpowered faction easily lifts some eyebrows.