Quirks govern the narrative of My Hero Academia, considering that they have become an integral part of society over time. These genetic factors are responsible for humanity's evolution in that they offer most people special abilities, each of which can be used in a different way.
As expected, Quirks come with drawbacks unique to them, which their users usually manage to surmount in order to effectively manifest their power. There are a few Quirks whose disadvantages are either severely imbalanced or simply impossible to overcome for several reasons—including their natural unwieldiness, birth-related issues, or the preset limits placed on them.
10 Aoyama Yuga's Navel Laser Is Ineffective During Prolonged Battles
Navel Laser is a massive burst of light that emanates from Aoyama's belly button. Its power is so strong that it can easily slice through rock and cement, allowing him to use his Quirk as an admittedly unwieldy sword.
The problem, however, is that Aoyama cannot control Navel Laser without his Sparkle Belt (like Cyclops' ruby quartz visors in the X-Men). In addition, he usually ends up with severe abdominal cramps if he activates it for extended durations, making his Quirk ineffective during prolonged battles.
9 Rikido Sato's Sugar Rush Is Heavily Limited By Time & Glucose Quantity
Sato's Sugar Rush Quirk magnifies his base strength levels by a factor of five, although he's limited by both time (a three-minute window) and utilizable glucose (10g at a time).
He needs to become considerably stronger before his Sugar Rush has a noticeable effect, but Sato has thankfully made some progress under U.A.'s guidance. Unfortunately, the absence of sugar causes hypoglycemic symptoms, such as tiredness, reduced perception, and so on.
8 Shota Aizawa's Erasure Results In Tired Eyes & Increased Sleepiness
Shota Aizawa's Erasure is one of the most dangerous abilities in the world, given that it can negate his enemy's Quirk(s) with a mere glance. He makes excellent use of it, combining his Quirk with his unique Binding Cloth technique to take on multiple combatants simultaneously.
It is imperative that Aizawa maintains his eyes in an open state as long as he wants Erasure to be active, which results in him being constantly tired (and sleepy). His Quirk is subject to the law of diminishing returns: the longer he uses it, the less effective it will be.
7 Manami Aiba's Love Can't Be Triggered More Than Once A Day
Manami Aiba, better known as La Brava, has a rare Quirk in that it is entirely supportive of other people and does nothing for herself. She often uses Love on Gentle Criminal to amplify his abilities, and the magnitude of her affections is directly proportional to the resulting power-up.
Manami cannot use her Quirk on more than one person, but the biggest drawback is that she can trigger it only once a day. It's easy to imagine Love's boundless potential if these hindrances were scaled back even slightly.
6 Neito Monoma's Copy Has Multiple Disadvantages
When Neito Monoma physically touches someone, he can replicate their Quirk and manifest it through his own body. He's been able to Copy Kirishima's Hardening and Bakugo's Explosion, but cannot do the same with Deku's One For All and Eri's Rewind due to the latter Quirks' "stockpiling" prerequisite.
Neito's Copy would have been overwhelming if not for its innumerable disadvantages—he can only use a single Quirk at a time, he can't keep the copies for more than ten minutes, and his physiology might not be capable of handling every ability he replicates.
5 Setsuna Tokage's Lizard Tail Splitter Can Be Used Against Her
Setsuna Tokage employs her Lizard Tail Splitter to incredible effect, showcasing her Quirk against Bakugo's team during the Joint Training arc. She can segment her body into a few dozen pieces, although there's likely a lower limit on size.
That said, the most notable inconvenience of Tokage's Quirk is that her separated body fragments lose their connection to her main self after an unknown amount of time has passed, a drawback that opponents can easily turn against her.
4 Hizashi Yamada's Voice Loses Potency Underground
Hizashi Yamada, or the Pro Hero Present Mic, has the strongest vocal Quirk in the MHA universe. His Voice can be projected at unbearably loud volumes and across great distances, making him a dangerous fighter at any range.
On the other hand, protracted use of his Quirk gives Yamada a sore throat, but this is something he has trained for years to keep under control. That said, the real problem appears when his opponents use subterranean techniques to surprise him, since Voice works best in the air and has little effect in other mediums.
3 Mineta Minoru's Pop Off Causes Scalp Bleeding If Overused
Mineta has earned his place in Class 1-A (from a power perspective and nothing else), manipulating his Pop Off Quirk into some truly spectacular Super Moves.
The "purple balls" that he plucks from his scalp have unbreakable adhesive properties, with only Mineta being immune to their stickiness. Then again, his head starts bleeding profusely if he plucks too much, a serious drawback to an otherwise fantastic Quirk.
2 Shin Nemoto's Confession Has No Combat Value Whatsoever
Shin Nemoto, one of the Shie Hassaikai's Eight Bullets, is blessed with a special Quirk that allows him to extract the truth out of anyone he wants.
All he needs to do is ask his target a question, and his Confession automatically ensures that the correct answer is obtained, regardless of whether the target is aware of it happening. Nemoto's Confession, however, cannot distinguish between literal and figurative responses to the questions asked, not to mention the Quirk has no combat value whatsoever.
1 Mustard's Gas Isn't Even Resistant To His Own Quirk
Mustard's Gas is one of the most drawback-riddled Quirks in the story—while he can produce soporific fumes and control them with great precision, he is himself unable to resist its effects.
Being immune to one's own Quirk is pretty much the most basic requirement for battle, or Endeavor's Hellflame would have engulfed him a long time ago. Further, Mustard's Gas doesn't have any features that prevent it from being dispersed with enough force, as exhibited by Kendo in the Forest Training Camp arc.