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.
"Miku Miku ni Shite Ageru" is the penultimate song of 2nd, and it's far easier than what came before, and what comes after.
"Continuing Dream" in F, very merciful for a (non-Extra) final song especially with the two hardest songs in the game coming before it.
F 2nd has the unintensive "The World is Mine" and "DECORATOR" sandwiched between two particularly brutal duos. "Two-Faced Lovers" and "Envy Catwalk" at first, and then "2D Dream Fever" and the infamous "Hatsune Miku no Gekishou" to round out the setlist.
Ear Worm: Too many to list. We are, after all, dealing with a rhythm game drenched in J-Pop.
Fake Difficulty: Any song with a PV where a lot of colorful movement is going around in the background, making it more difficult than it should be for the targets and incoming beats to be seen.
Fridge Horror: Miku may be a tyrant in Sadistic.Music Factory, but she's also doing what she does simply for the sake of survival and escaping the factory (not getting the Chance Time bonus) means leaving her to starve to death!
Les Yay: Miku and Rin have a lot of this in the bonus materials for the games.
Nightmare Fuel: The music video of "Sadistic.Music Factory" in F. The song itself could be pretty scary if one takes into account the lyrics (which are about being trapped in a factory and forced to make music for a mechanical tyrant for the rest of one's life) and the frightening robotic monotones, but combine all that with the Chance Time ending...
The Disappearance of Hatsune Miku, which has the song become garbled, as well as there being interference and odd glitches in Miku's movements, constantly. According to Word of God, Miku is being deleted while attempting to complete the song. The end of the song is when she finally crashes due to it.
Toward the end of Disappearance, Miku can be seen banging at the screen with her fists as though begging the player to help her. And at the end, she says a cheery goodbye before just...freezing up with a sad smile on her face as her voice speaks eerily in the background.
Scrappy Mechanic: The star links in F 2nd aren't proving to be very popular. Their timing is often inconsistent with the rest of the song, either much faster or much slower, which throws a lot of players off on their first try. As well as many, many tries after that.
Chance Time in the original PSP game can feel like this at times as missing a single note on it in several songs is what could very easily separate a Standard and Great score.
Tastes Like Diabetes: The inclusions and actions of the chibi Vocaloids in "God-Tier Tune" make it the most adorable apocalypse ever.
That One Level: "Sadistic.Music Factory" and "Negaposi*Continues" in F. The beat is fast and the targets come almost constantly. You have to be on top of the rhythm the whole time, or else you'll find yourself failing out very quickly.
Pretty much any cosMo song qualifies. In addition to "Sadistic.Music Factory" mentioned above there's also "Hatsune Miku no Gekishou" in 2nd and the famous "The Disappearance of Hatsune Miku" in the first game and extend (more so in the Miku Flick spinoff).
The Project Diva F 2nd version of "Hatsune Miku no Gekishou" is, while not quite as difficult, removing all the longer chains, still tricky to master.
A new challenge item removes the short delay between a note arriving and needing to be played, meaning you'll fail a song, guaranteed, on your first try purely since there's no warning that you need to play the next note.
Woolseyism: Several instances of this in F. For example, one song that would literally translate to "Left-Behind City" was named "Urbandonment", and "Remote Control" became "Remote Controller", which is appropriate given the video has the Kagamines controlling each other through their devices.