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.
Bermuda, despite being a Cursed Arcobaleno, goes down by Tsuna's new Super Mode in one chapter. Immediately after this, Checker Face reveals himself to be a Well-Intentioned Extremist and gives into Tsuna's peace negotiations.
Byakuran despite all the build-up, especially being a Multiversal Conqueror of almost every multiverse in existence except the one Tsuna lives in and having all the knowledge and strength of those worlds, gets defeated swiftly in six chapters that reeks heavily of Deus ex Machina.
The only way to explain what happened to Adelheid Suzuki: she went from fighting evenly with Hibari to not being able to land a single blow on him within slightly more than a week. This is especially egregious when you consider that not only did she get a massive power boost from the "Vongola Sin", it was expressly stated that this power would continue to grow over the course of the next seven days.
Tsuna goes through this in every single arc. He manages to defeat the Big Bad from each arc, but in the next one is stomped by the first enemy he faces and feels completely insecure about it.
Expectation Lowerer: First shown as being slightly weaker then Squalo, Basil is this for Tsuna, Yamamoto, and basically everyone else.
Fashion-Victim Villain: Daemon Spade. Which is sad because for the most part he applied to the Evil Is Sexy trope... and then he went and possessed Mukuro. Now we have this. Standard fandom response is laughing your ass off.
Growing the Beard: By the Varia arc; the Kokuyo arc was the first to be Darker and Edgier, but the Varia arc did wonders in world-building and generally having a more engaging plot.
Harsher in Hindsight: Reborn's early interactions with Lambo. Sure, we know how annoying Lambo is, about all that "rivalry" thing and that he's not gonna be killed even by grenades, but once we realize it's an adult hitman beating the living crap out of 5-year old kid... Dude, Not Funny!.
Byakuran definitely fits the bill. In every single one of the multiverses.
With the reveal of who really masterminded the time travel plot, TYL!Tsuna gets props for putting the kids in significant danger just to motivate his younger self. His own younger self can't believe it was really him.
Depraved Homosexual Lussuria of the Varia. Who during his fight with Ryohei expresses blatant interest in him while simultaneously reveling himself to be a necrophiliac.
Dino's a more arguable case, due to his grin when you see this first encounter with Hibari as his pupil.
Glo Xinia. It is heavily implied during his encounter with Chrome that if Mukuro hadn't stepped in when he did, Glo probably would have tried to rape her.
Spanner's wildly regarded as a pedophile due to his interactions with Tsuna, which included stripping him for no real reason and handcuffing him to his bed. His blushing face (in regards to the X-Burner technically) and sudden Heel-Face Turn don't help.
Byakuran smiles constantly, loves "symbolic flowers", kills off one of KHR's other memetic rapists, and has signature moves where he 'penetrates you'. Oh and let's not forget his image song where he asks you to 'give him your body'. Then there's his blatant obsession with Yuni once she escapes from his grasp. Although there is a good reason for it, the vibes are still there. Not to mention his grabbing of Tsuna from behind and whispering into his ear...
Julie Katou wants you to know that you have "date plans" with him. And he will help you dress for them~ Shame this is all due to Daemon Spade possessing him.
Daemon Spade's a pedophile that wants your body. Nufufu~How ironic that one of the Memetic Molesters above is a VICTIM for this guy.
Despite the clear amount of drama and "Oh Crap" moments going on in the most recent chapters of the Inheritance Ceremony arc, the Fandom still laughed its collective ass off at how Daemon looks in Mukuro's body.
There were many who were Demoted to Extra in the final arc, such as Ryohei, Lambo, and even Yamamoto and Gokudera. Especially jarring since they had just received brand new Cambio Forma weaponry in the previous arc, which Gokudera and Yamamoto had barely shown off.
20YL!Lambo, the mature, handsome, and incredibly Bad Ass future version of 5-year-old Lambo, only ever appears in one chapter. It's justified that he can't show up in the Future Arc due to Lambo not being in possession of his 10-Year Bazooka, but even afterwards it's only 10YL!Lambo that makes a reappearance.
The Arcobaleno Arc could've been great, if it wasn't for its premature ending.
Mukuro and Hibari never got their rematch despite their build up.
Uncanny Valley: Bermuda's true form resembles a doll whose proportions are too unbalanced. His eyes are too large for his baby like face, has a nose that is practically nonexistent and has the arms of a bodybuilder attached to an adolescent body.
Chrome is the strongest female in the series; a girl who lacks organs, depends on a man to help her fight, and finds herself helpless and/or captured in almost every single battle. She is also the only girl that has been fighting the bad guys in the last two arcs.
The poor treatment female characters get has been noticed by several people. Kyoko and Haru are probably the most abused. Whenever the main characters are dealing with a serious conflict, Haru and Kyoko are not even told about it and have stay at home doing things for the boys (alongside I-Pin and Bianchi, even though they're both professional assassins). The thing is that this never changes so many (even their fans) saw their absence from the Simon arc onwards positive since they would not be used so much again.
Adelheid at least started out to be what looked like the first genuine Action GirlBadass of the series. People were even describing her as the Foil to Base Breaker Hibari during her shining moments. Sadly, it all went away the moment she was forced into the background to rot for the rest of the manga after the Daemon Spade reveal.
Lussuria's character. He's the only cast member who's explicitly homosexual... and made moves on an underage boy while revealing himself to be a necrophiliac.
Wangst: A strange case. Tsuna's life is supposed to have gotten better after meeting Reborn, but Byakuran tortures him in the Future arc by saying how much his life sucked in that arc. Ironically, he is kind of right considering how at the beginning Tsuna was exasperated with the idea of not going home while his friends were trying to stay calm about it.
Gokudera, especially when you learn about his past.
Chrome also due to the hell she goes through. Her parents abandoned her to die, half her organs are missing, she's constantly belittled, and most recently she's been hypnotized and tortured by D.Spade purely to push Mukuro's Berserk Button.
20YL!Lambo, particularly when you realize, thanks to Byakuran, exactly what kind of future he has to return to.
Yuni, and by extension Luche herself. Luche was Blessed with Suck, received the less-than-beneficial end of the Cursed with Awesome stick shared by her other partners, and worse yet has to pass that on to all her future descendents.