One Piece introduced audiences to the Devil Fruits, fruits that grant power but at a great cost to its nautical heroes and villains. Losing your ability to swim aside, for many fans, the prospect of gaining rubber powers, being able to manipulate the elements and devour the souls of those in your grasp might sound like an appealing prospect. Indeed, most Devil Fruits sound awesome. In the fantasy pirate world of One Piece, each Devil Fruit can potentially turn you into one of the most feared pirates on the Grand Line.
In a realistic setting, however, we have to ask if these powers are all that practical. While many Devil Fruits sound worth the cost, others, like the SMILE Fruits, are just plain awful and others grant powers that honestly might cause more trouble than they're worth. But which of the Devil Fruits in One Piece would absolutely suck in real life the most?
The Swim-Swim Fruit
Most Devil Fruits in One Piece makes it impossible to swim in the water, and while this one is no exception, the Swim-Swim Fruit grants its user the ability to swim through solid mass. Specifically, the Swim-Swim Fruit, consumed by Donquixote's Senor Pink, allows whoever bites into it to power swim through the ground and out through walls.
While this might sound like an interesting power, it also means that the user is completely submerged in the ground, so they might accidentally swallow some of the ground they swim through. This also means the person using it might consume bugs, animal droppings and any toxic chemicals that might've spilled into the soil. Worse yet, if you are knocked out mid-stride, you would sink into the Earth, drowning in matter that would solidify around you as you lose control.
The Glint-Glint Fruit
Few Devil Fruits in One Piece are as powerful as the Glint-Glint Fruit. The Glint-Glint Fruit is a Logia-Type, one that allows anyone who consumes it the ability to convert their body into light energy. With it, they can move their bodies at light speed, generate lasers and even barrages of lethal bursts of light. Its user, Admiral Kizaru, is able to manipulate and control the light perfectly due to his incredible reflexes. However, any ordinary human being might find the Glint-Glint Fruit too much to handle.
If an ordinary human traveled by light, they might find themselves overshooting their destination. Light travels about 186,282 miles per second. In one second, light can circumnavigate the Earth, which is a relatively small 24,901 miles, seven-and-a-half times. Sure, Admiral Kizaru can accurately stop himself just in time to reach his destination, but if a person failed to react fast enough, they might find themselves rematerializing over the Atlantic Ocean -- which, for a Devil Fruit user, is suicide.
However, things get a little worse when you take into account physics. The closer physical mass comes to light-speed, the more that mass increases. If a physical object of any size reached light-speed, its mass would become infinite. That means that literally everything it touched or passed near along its path would be obliterated. It would be as though a black hole skimmed the planet's surface. Simply put, traveling at light-speed would be enough to destroy the whole world.
The Munch-Munch Fruit
One Piece has one Devil Fruit that takes the phrase "you are what you eat" to its logical extreme. The Munch-Munch Fruit allows its user to manifest and merge anything it eats into his body. It is possessed by Wapol, the main villain of the Drum Island Arc, and, even in-canon, the Fruit has some negative side effects, such as constant hunger.
However, more horrific than that, the Munch-Munch Fruit essentially guarantees its user will be subjected to horrific transformations on a regular basis. Constantly hungry? Well, that means you'll be constantly consuming food that could manifest in horrifying ways. To make matters worse, in order to maximize the use of this ability, it means you're going to have to eat metal. Wapol consumes the bulk of Drum Island's weaponry in order to gain immense power. Are you really ready to eat guns, bullets and metal shields?
The Smooth-Smooth Fruit
The Smooth-Smooth Fruit is certainly one of the more disturbing Devil Fruits in One Piece, though not for reasons that are immediately apparent. The Smooth-Smooth Fruit allows its user to become extra slippery. Anything that comes in contact with them simply slides right off. However, one unusual aspect to this is that excess fat also just slides off a person's body, as seen by the Smooth-Smooth Fruit's user, Alvida.
Here's where things get odd. On the one hand, the Smooth-Smooth Fruit makes someone incredibly slippery by default, which means if anyone were to try hugging or kissing you, they'd slide right off, whether you want them to or not. However, the idea that your body fat would slip off your person is somewhat grim, like something out of a Cronenberg film. What happens to said body fat? If you ate the Smooth-Smooth Fruit in your living room, does that mean you'd have to clean up chunks of fatty, pulsating meat from your carpet? Extreme weight loss can cause serious health issues, which this Fruit does not seem to negate.
The Dark-Dark Fruit
Unsurprisingly, one of the most powerful Devil Fruits in One Piece is also arguably the worst one to have in real life. The Dark-Dark Fruit is the Devil Fruit wielded by Marshall D. Teach. By its very nature, it stands out as one of the evilest Devil Fruits out there. It allows the user to manipulate the darkness itself, creating sentient darkness that can absorb everything around it. Any attacks or elements can be nullified with this living darkness and any Devil Fruit powers can be negated, so long as Teach touches them.
Of course, while all of these powers in-text have weaknesses (the darkness doesn't make the user invulnerable to other attacks, for example), the real concern here is the darkness itself, which sounds like a black hole -- one of the most powerful wells of gravitational power around. The Dark-Dark Fruit allows the user to bend and control gravity with this darkness.
All this seems to indicate that anyone who uses the Dark-Dark Fruit is playing with a gravitational force powerful enough to consume and devour stars. All it would take is for a user of the Dark-Dark Fruit to lose control for a second and everything could be obliterated. The whole planet could be swallowed up by it, taking everyone and everything into it. It's a shock the world of One Piece hasn't fallen into the seas through Teach accidentally summoning a world-ending black hole in his sleep.