Anti-Climax Boss: Well depends if you think Hammer Man should still be a Robot Master or not. Prototype Zero is also, if you consider him to be the final 'boss' of the game. A letdown compared to the three-part battle in Wily stage 4.
Much debate has gone about this, but according to at least 1 forum post and the mock-up video of Tank Man's stage, his level may be the EASIEST to get through. The final design of the level is quite a bit harder than shown in the mock-up video, but still one of the easier levels.
The final level, even after accounting for its handicaps.
Individual players either come to praise the game's daring approach or despise it as an exercise in frustration due to the brutal difficulty.
A number of players also dislike the art style and certain songs, while the rest find them good.
The ending's take on the events that bridge the Classic and X timelines. Some think it's really awesome and love the connection, others don't mind it, some feel it could be handled better and there are those who consider it an untouchable plot point and felt offended by the game's pretentiousness. Others still feel that having the game end with an unwinnable boss fight ends the game on a sour note.
Checkpoint Starvation: One of the most frequently stated complaints about the game, though in most cases it's not the lack of checkpoints so much as where they are. For example, the checkpoints in Yo-yo Man and Trinitro Man's stages are set at about 1/3 of the way through the stage, while the checkpoint in Glue Man's stage is considerably further than halfway through the stage.
Fridge Brilliance: In Rainbow Man's stage, aside from the prism enemy, the only other enemy is a leprechaun in a pot that tosses out gold coins. Why's that? Well what do you find at the end of a rainbow?
Game Breaker: Holy Hell, Prototype Zero. You have to have already beaten the game already on original or instant death, so it can't be used to cheese the game as soon as you start, but once you unlock him, he bends Unlimited's legendary difficulty over and smacks it acrossed the rear. Granted, he starts out weak, and grows more powerful the more bosses you beat, but even if you only beat a third of them, he completely punches a hole through a good portion of the games difficulty. Honestly, if you're still steaming after finally beating the game as Mega Man, obliterating the game as him really alleviates some stress.
Goddamned Boss: The second Wily Machine. Its attacks are fairly easy to dodge, but its vulnerability window is tiny, and the Rainbow Beam, the most effective way to hit it, does barely any damage.
Jet Man spends most of the fight offscreen. While all but one of his attacks can be easily dodged, trying to kill him with the Buster takes forever only because he's never available to hit.
Good Bad Bugs: The top of Trinitro Man's head is intended as Schmuck Bait as hitting it causes a lot of damage but hitting it three times causes it to explode. If, however, the 3rd hit's damage is enough to KO Trinitro Man, the head does not explode and you win.
Using Glue Man's weapon, you can zip through walls similar to the bugs in earlier classic Megaman games. It was patched out in a later version, but demand caused the devs to release a "Speedrunning version" that reinstated the glitch (along with adding a few features, like an in-game timer).
Hell Is That Noise: The screeching sound that plays when the boss of the first Occupied Wily stage shows up.
Yoku Man's stage will make you dread the 'bwoom, bwoom' of yoku blocks.
In Zero mode, you play as a rogue prototype robot who constantly lose health, and the way to stay alive is to kill enemies. In Mighty No. 9, Ray's gimmick is exactly that. It also helps that she's an Expy of said spoiled character.
"Holy Shit!" Quotient: Surprisingly high. Highlights include Proto Man getting infected and turning on you, Bass going rogue, and the fight with Prototype Zero in the finale. The ending is one big holy shit moment since it essentially bridges the Classic and X series.
Hype Backlash: After coming out of years of development, the game's extreme difficulty level and other issues weren't received too well by a number of people.
Memetic Mutation: "So, like a normal Mega Man Unlimited level." Explanation It's become a meme from both detractors and people who like the game to say this in response to Marathon Levels in other games, either by describing the level in detail or using it in response to someone talking about a Marathon Level in another game, the joke being that all the levels in Mega Man Unlimited are Marathon Levels in and of themselves.
Never Live It Down: Despite the many patches and updates made to make the game more balanced, Unlimited is mostly remembered for it's Checkpoint Starvation and odd checkpoint placements, such as putting a few before the mid-bosses of most levels note Granted, only the later of which got fixed and readjusted to better places. The former is only fixed if you play on easy mode and being brutally difficult, often verging on Fake Difficulty at times. note In addition to the fact that most of the more blatant Fake Difficulty areas got majorly adjusted, many argue that most of the stuff people complain about also holds true for any Mega Man game, being hard when you first play it, but gets easier on repeated playthroughs. Most often point out stuff like death spikes are mostly aesthetic and you'd have to be an idiot to die from them, and while Rainbow Man's stage does have far more instant death beams than Quick Man's, it also gives you far more leeway on them to react than Quick Man's.
Pacing Problems: The sheer amount of things you have to do in Fortress Stage 4 causes this on its own, let alone when considered alongside the "level" that follows it. There is also a feeling among a Vocal Minority that most of the stages are too long to have only two checkpoints.
Pandering to the Base: The development of the game itself is a form of this, due to the fans begging Mega Phil X to make a full project out of his fan-made trailer. Although, Phil is a dedicated Classic Mega Man fan, so it evens out somewhat.
Yoku Man has an attack where he will make death spikes appear one tile above the ground in a row, except where he is about to appear and right beside him. If you didn't make it through his previous attack pattern properly, you're almost certain to die, unless you get lucky.
Jet Man's bombs. If you're on the right side of the screen, you get about a second to dodge or counter the bombs (and they don't fall straight, unlike the bombs dropped by the regular enemies). If you're on the wrong side, then you're just screwed. This attack gets an upgrade with his Omega form where he carpet-bombs the whole arena, giving you less room to dodge before they explode. Jet Man's other attacks can be dodged consistently, but this attack alone makes him a borderline That One Boss.
The Captured Wily Stage 1 boss is most certainly abusive with his electrifying ladder attack. One hit while on the ladder it affects is enough to send you into a bottomless pit.
Its other attack is annoying as well, since one hit from it will also ensure your death.
That One Boss: Most Let's Players agree or seem to have the most issues with Glue Man. Most have a hard time keeping up with his fast pace. He also deals a lot of Collision Damage. His Omega form is even worse, as he now shoots three glue shots at a time as opposed to one, and his collision damage has been increased.
Jet Man can also pose quite a challenge. Not only is he quite fast, but his attacks all cause small explosions, come quickly, and do lots of damage. He also doesn't give you many opportunities to hit him.
Yoku Man. He summons yoku blocks, then teleports to a random yoku block and fires a fast moving projectile at you. After three yoku blocks, he teleports to the bottom of the stage and fills it with spikes, except for a small area in front of him. On the bright side, if you got Yoku Attack prior to the fortress, you can serve him a taste of his own medicine in the Boss Rush. He's absurdly weak to his own weapon.
The crab boss at the end of the first fortress level is also aggravating. You fight it on a set of three ladders over a Bottomless Pit, where getting hit by anything is almost guaranteed to be fatal thanks to the Knock Back. Coming in with a stock of Beat Whistles is highly recommended.
The fight with Bass, and then the two-form Wily Machine. None of the fights are particularly hard, per se, but they come one after another with no chance to regain lost health or weapon energy without an E- or W-Tank.
Rainbow Man's level, which is loaded with Quick Man death beams. It even gets worse, as some of the beams will pass through prisms that fan the light out into a wide area (that isn't any less deadly). It got toned down as of version 1.1.0, cutting out a number of potential cheap deaths, though the stage is still quite challenging.
Captured Wily Castle Stage 4 is a prime contender, even accounting for the fact that it's a Fortress stage. 9 Robot rematches complete with gimmick mini-levels prior to each rematch and a 3-part final boss battle all in one stage. It is enough to make players rage.
Jet Man's stage, which involves a long portion over a bottomless pit, which you must cross using platforms that move when you run the opposite direction. This is while steady streams of enemies jump up from the bottomless pit, and a pair of enemies perform what can only be described as an air raid. If you get hit by any of these, especially in midair, instant death is pretty much guaranteed.
Tough Act to Follow: Since the game came out years after the release of the well-received Mega Man 9 (which Unlimited was initially intended as a follow-up to), this was bound to happen. It doesn't help that Mega Man 10, which came out during its development, looks like a more well-balanced game in comparison.