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.
In Mii Force, Brown, White, and Light Blue shirts are better than other shirts. Brown and White can destroy purple plasma shots while Brown also gives a perpetual shield against physical attacks. The only drawbacks of Brown is that when it blocks a physical attack it launches you in the opposite direction, potentially into another trap and Brown shirts can be really hard to find. Light Blue shirts just have good range, rapid fire, damage, and destroy fire based attacks. While other shirts like Blue, Red, and Orange are good in general and other shirt colors are good in certain levels (2-3 for Black, 3-1, 4-1 and 4-3 for Light Green), Light Blue, White, and Brown are the best shirts to have on your ship during most levels of Mii Force. They make arcade mode a bit easier. The only useless power is Dark Green. See Tier-Induced Scrappy for details why.
In Find Mii, Yellow, Light Blue, Dark Blue, Red, Black, and White shirts are often required for passing certain levels. In addition, Red and Dark Blue magic is handy for armored enemies where, if not strong enough, can still deal damage that way, and Light Blue and Light Green magic can be handy against the Green Slime.
Demonic Spiders: Any enemy that takes very little damage from your attacks, or uses any type of magic (e.g Fortifying Magic, Mirage Magic, Level Down Gas, Nullifying Magic) in Streetpass Quest 2/Find Mii 2.
Mummies are especially infuriating since they usually cost you a hero before you even do anything.
Certain Reaper enemies will spew Cursed Gas, which often prevents your heroes from acting at all. A Dispelling Draft might as well be mandatory.
HomePassing (see the main page) and Play Coins. The latter has an exception on Puzzle Swap, see Scrappy Mechanic below.
In Find Mii 2 using Potions of Power, Dispelling Draft, and Team Attacks can pretty much destroy enemy gas effects and be enough to destroy tough adversaries without too much trouble. If you have an Orange Mii (or better yet two Orange Miis team up) and use magic to give their teammates extra attacks, combined with using other Miis to use Vial Of Valor to bring back people who have already attacked, even the Final Boss can be defeated easily.
Light Blue Miis in Mii Force: Even when low levelled their attacks are extremely effective against swarms of enemies and defending against attacks.
Love to Hate: In Mii Force, Gold Bone may be a jerk, but at least he's entertaining.
Memetic Mutation: Creating deformed Miis of Nintendo characters with slightly-off names. The most famous examples include Lonk◊ and Petch◊.
Rescued from the Scrappy Heap: Brown Shirts weren't that useful in Find Mii, as they could only summon another hero, and it was always a random color. But in Mii Force, they have a drill weapon, handy for defense.
Some of the potions you order in Find Mii/Streetpass Quest 2 may turn out to be defective, wasting your hero and your Play Coins. Thankfully removed in the update that included the additional games, considering they're by far the most expensive thing that consumes Play Coins in any of the games outside of hiring additional troops in Warrior's Way.
In Puzzle Swap, using Play Coins to obtain pieces is not a surefire solution to a dearth of Street Passers - the game can (and often will) give you pieces you already have, sometimes several in a row. This can become extremely aggravating when one repeatedly gets dud pieces on a panel with only one piece missing.
Stop Helping Me!: Mr. Mendel quickly falls into this. After playing the game for a while, many players will be shouting at Mr. Mendel to just shut up after repeating the same piece of advice countless numbers of times and just wanting to get to greeting the guests. It's even worse after one has grown all 80 plant breeds, because he'll still offer advice on how to grow new breeds. This is also unpleasantly averted by the fact that he stops wondering if any of your plant pals will come by, forcing you to memorize or write down the Miis that you've Street Passed with more than once.
Beating the first game with 30 or less Miis is going to be very difficult if you don't get the right color characters, especially since you can't team up in that game.
Level 3-3's fourth gem; you have to destroy both of the green Ferris wheels at the same time. Try doing this when there are a ton of projectiles flying at you.
The Level 4-3 third gem is notoriously hard to obtain; during the descending sequence, both of the huge submarines must be destroyed at the same time. It's said the best way to pull this off is to use three level 3 Light Green turrets, colors that are very hard to come by without spending Play Coins.
Destroying both of the Final Gold Bone robot's hands is insanely difficult without the right weapons, and even then it's tricky not to keep getting hit while trying to destroy his flying fists at the same time you attack.
Obtaining the "battle 300 visiting monarchs" plaza ticket. Depending on where you live, this can be almost impossible to achieve. And even in an area with lots of 3DS users, it can still take a very long time.
Emperor Fynalle in Warrior's Way has 8 times as many troops as you do you when you first face him, you can't spy on him to find out what kinds of troops he'll use, and the match goes on for 5 rounds instead of the usual 3.
The final fight with Gold Bone in Mii Force's Arcade Mode is this, unless you somehow still have most of your weapons with you.
Find Mii/StreetPass Quest has a lot of these, mostly revolving around finding a Mii with the right ability at the right time. An update has allowed you to hire past heroes, however, alleviating the problem provided you've found at least one Mii with the needed shirt color. Even so, there are still some hair-tearingly frustrating rooms:
Colored shields block all sword attacks except from heroes with a shirt of matching color. Magic goes around the shield, but at half damage. God help you if you don't have a friend to change their shirt into the color you need. The worst room of this kind is the first Forest of Sorrow room in the Secret Quest of Find Mii/StreetPass Quest 2. It contains two green slimes, which cannot be damaged by magic and have absurdly high evade rates against sword attacks (but only 3 HP), and one of which is defended by a shadowlight shield, which requires a black/white team to break.
Rooms covered in darkness can ONLY be passed with White magic, forcing all other heroes in front of the White hero to flee immediately upon entry. Unlike shield rooms, you can't re-order your heroes to put White in front. And no matter how many heroes you hire, if none of them are white, every one of them runs away and no progress is made. At least you never have to light the room again on that playthrough once you light it once.
Streetpass Quest 2/Find Mii 2 goes beyond the frustration of darkened rooms with fire rooms requiring two Dark Blue heroes to put out the flames, poison rooms that require two Yellow heroes to send away the poison, and ice rooms that need two Red heroes to melt the ice.
The Sealed Passage in Find Mii 2's Secret Quest. It contains a powerful Mummy enemy and the Reaper General. The Mummy will scare away your Miis while the Reaper spews curse gas and has a defense of 2. And although you might think it would be best to get rid of the weaker Mummy first to save your Miis, if you do, then the Reaper will REVIVE it with full HP! (Useful hint to remember here: White Mii Magic causes extra damage to reapers)
The Ransacked Ruins/Dolorous Deepwood route through the Secret Quest in Find Mii/Streetpass Quest 2 could be described as That One Route. It features two rooms with enemies behind yellow shields, two poison rooms requiring two Yellow Miis each to get past, a room with two green slimes (one behind a shadowlight shield), a Rampaging Mummy (guaranteed to scare away the first Mii that enters the room), and the same ice room followed by a darkened room with a curse gas-spewing Reaper Commander that lie at the end of every route.
If you follow the Diabolical Altar/Ominous Shrine/Sealed Passage route through the Secret Quest, then you will have to fight, in a row, a Roaming Mummy and a Marauding Mummy (one of which will likely scare away the first Mii), a Rampaging Mummy (60 HP, guaranteed to scare away the first Mii), a Golden Golem (defence rating of 2 and 65 HP, so a OHKO is only possible with a Level 15 team and support magic) in a frozen room, a Reaper General (defence rating of 2 and 75 HP, making a OHKO difficult even with a Level 15 team and support magic; also spews curse gas and will completely revive the Marauding Mummy in the same room if you kill it first), a Diamond Golem (defence rating of 2 and 85 HP, making a OHKO nearly impossible), a Reaper Captain and a Marauding Mummy in a frozen room, and a curse gas-spewing Reaper Commander and Armored Demon in a dark room.note The last three are part of every route in the game, but are bad enough without the long parade of frustrating enemies that precedes them in this route.
Bone Rattler, the roller coaster level in Mii Force, makes the game into a literal rail shooter, only letting you move vertically, and there are two forks in the path. Especially annoying in that you must take the hard routes in order to get some of the treasures.
In Flower Town, completing any job with a specifically colored flower. Unless you get very lucky, you'll need to plant the seed with another Mii with the appropriate shirt color, which may require a lot of patience if it's a less popular one. Completing any job requiring a specific height of flower (especially for gold medals) is even more frustrating, as obtaining a suitable seed requires pollination from a flower of the appropriate height, and even then it is not guaranteed that it will grow into the correct breed at the correct height.
Dark Green Miis, whose abilities are practically useless in both Find Mii and Mii Force. In Find Mii, they double your next warrior's level. It sounds nice but unless you have a bunch of level 1's and the Dark Green is in front then it's worthless. In Mii Force, Dark Green gives a bouncing ball which does bad damage unless bounced off an object, has bad attack radius, and offers no defense like Brown or White shirts do.
In Warrior's Way, people who have no street passes, thus their army consists of a single useless soldier.