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.
Accessory-wise, we have The Narukami Bracelet/Thunderous Bangle, which completely prevents ALL Spirit Consumption as long as it's worn. Since you get it for beating Total Pandemonium/Night of Absolute Chaos, it's almost a Bragging Rights Reward... Except the game is skills-based, and you can never become so powerful that the game becomes a walk in the park, though being able to completely negate all damage via blocking with no risks certainly helps. Another Game-breaking accessory is the Nio Bracelet/Deva King Bracelet you get from the Dragon God(Kisuke's seventh boss) in the post game. This item adjusts the attack strength of all swords to a static 700, which becomes brutal when combined with Specials like Gale II. Only two or three swords are made worse by this item, and those are of course the very final swords obtainable, so the strength-loss is a moot point, and it's very helpful for aiding lower-level characters in cheesing past the Night of Absolute Chaos at an earlier level than the game recommends.
The characters in the Genroku Legends also have Game Breakers, particularly the very final Skill obtainable in their Training; Okoi/Miike gets a Skill which prevents her from losing Spirit as long as she's blocking on the ground(which may be because she's a Tortoiseshell Cat), Gonbe gets a Skill which instantly refills ALL of his Spirit Gauges upon doing an Area Attack(his most reliable weapon, the Sickles, can break from overuse, which immediately grants use of an Area Attack... Which will refill the Sickles' Spirit Gauge, enabling for nigh-infinite spam), Arashimaru gets a Skill which makes him completely and utterly immune to damage as long as Shirohebi is out(ie the entire duration of his Midair Special Arts), and Rajyaki gets a skill that deals bonus damage and instantly refills a big chunk of her Demon Form gauge whenever she deals a Crushing Blow (which can be quite easy depending on which moveset it's equipped on, and the Demon Form does huge damage while being completely invincible, so spending as much time in it as possible is very powerful).
Goddamn Bats: Pheasants and poison moths. Tengu can also be pretty annoying, since they usually show up in areas with differing elevation, which makes chasing them down or avoiding their attacks a pain.
Kite ninjas as well, as they love to fly just out of reach while pelting you with shuriken and bombs, and have a nasty habit of blowing up in your face when they die.
MST3K Mantra: During the reign of Shogun Tsunayoshi Tokugawa, it was illegal to be a prostitute, sell silk, hire waitresses in your restaurant, or hurt animals. The game pretty much only paid attention to the "hurt animals" part. Justified in as much as these things are part of the Jidai Geki feeling it is attempting to emulate, and their loss would make for a poorer game.
Nightmare Fuel: Some of the bosses are truly awful-looking. Especially the giant centipede. *shudder*
Inugami is disturbing as hell.
Polished Port: The Vita version adds a lot of improvements over the original Wii version. Aside from DLC characters and even prettier HD graphics, the presence of an actual jump button instead of the awkward up button to jump, as well as the ability to map the dodge roll to a shoulder button instead of having to do a quarter-circle motion, improves the gameplay immensely. The localization was also handled by Aksys (the same localization team of the BlazBlue series), with an eye toward a more faithful translation than Ignition's widely disliked version.
Prop Recycling: The scenery is very nice to look at, but be prepared to see the same screens over and over.
Scrappy Mechanic: Enemy scaling rears its ugly head. You can be fifty levels above the recommended minimum for a lair, with thousands more HP and ten times more attack power than you're 'supposed' to have, but they still hit hard and soak up the punishment you deal out. So fighting random battles doesn't do you much good... but you have to level up because there are minimum stat levels on the swords.
Ugly Cute: Ippondatara's boar form. When you're fighting him he just seems like some regular monster, but then you beat him and he asks you to spare his life in exchange for information. Gets even more cute when he explains that he transforms into the giant monster you fought, because he wants to protect his hot spring from humans. He tells you this while bathing in the hot spring himself with Momohime and Kongiku.