Pyrovore
Pyrovores are a Tyranid unit widely considered to be the worst unit in all of Warhammer 40k. They are representative of Robin Cruddace's awful, awful Tyranids codex, in which at least half of the units have absolutely no idea what they're supposed to be doing or are so abysmally awful at what they are supposed to be doing that no one ever takes them--and Pyrovores are the worst of the worst.
Fluff
Pyrovores are living fuel refineries for the Hive Fleet, they gain said fuel by eating. They can pretty much eat anything, from organics to inorganic material like rocks or metal, which they're able to digest thanks to the corrosive acids inside their stomatchs, which explodes if they get bumped too hard.
A Pyrovore has two brains, one for that lumbering hunk of fail that eats like an obese kid and another for the giant flamespurt cannon on it's back. This allows the Pyrovore to devour everything like a neckbeard eating meatbread while the cannon keeps watch. The funny thing about this is that it's cannon is much smarter than the beast itself, the Pyrovore is only driven by it's need to eat while the cannon's smart enough to target enemies on it's own. This might very well explain why the Pyrovore's terrible in meele combat.
Why the Hive Fleet thought it was a good idea to put a derp-grade brain into a beast that needs a modicum of intelligence to handle itself in combat, especially given it's ability to explode, while putting the smart brain on a gun, which really isn't that powerful compared to the other races, and make them incapable of working together, begs the question: why?.
How Bad Is it?
Pyrovores are worse than Old One Eye, a DISTRACTION CARNIFEX special character in a codex that hates DISTRACTION CARNIFEXES, who is renowned for costing MORE than a Land Raider and dying to a stiff breeze... but at least he has a general idea of what the fuck he's supposed to be doing and would be tolerable if he was cheaper.
Pyrovores are worse than Chaos Sp... uh, that one Chaos unit everybody thought was terrible... but at least they've seen a Phil Kelly-granted buff in the newest Chaos Space Marine codex.
Pyrovores are worse than Fire Warriors in melee, who are unceremoniously chopped up in seconds... but at least that's funny, and things have gone wrong when squads assault Firewarriors, and Firewarriors can do that "Overwatch + Markerlights + other squads nearby can Overwatch for them + Markerlights", and can kill Land Raiders in melee with Haywire Grenades.
Pyrovores are so bad that fluff writers and writefags try to forget that they even exist. In fact they appear only in one BL book (The Last Ditch) and they are as terrible there as on tabletop (like accidentally blowing apart a hive Tyrant and his guards and getting mocked for being terrible in universe). Which we choose to take as further proof of Sandy Mitchell's awesomeness by taking the now memetic godawfulness of the Pyrovore and making it canon.
Pyrovores are so terrible that why they are even a thing remains a point of debate because holy shit who in their right mind would think that this...this....thing...was a good idea? It serves no conceivable purpose and there are much better platforms for a flamethrower in the Tyranid bestiary than the body of an indirect fire mortar carrier never intended to even get in visible fucking range of the god dawn enemy. I mean fuck, there's even one in the same motherfucking Codex, all you'd need to do for the Tyrannofex is light it's acid spray on fire and then BAM! You have a flamer on a nonshitty platform. Or Warriors or something, because they're basically crapsack Terminators and Terminators get a Heavy Flamer! Or just, anything!
Why are they so bad?
The biggest problem comes down to the fact they're in the Elites slot, which is easily the most jam-packed slot in Tyranids. They're competing with Hive Guard (probably the best anti-tank infantry in the game), Zoanthropes (incredibly powerful psykers that can throw down an ABSURD amount of mental dakka each turn), Venomthropes (an excellent support unit that provides mobile cover, defensive grenades, and hilarious 2+ Poisoned attacks), Ymgarl Genestealers (basically the only unit in the game that can assault the turn they appear), the Doom of Malan'tai (an incredibly cheesy unit powered by the tears of Eldar players), Deathleaper (perhaps the best character assassination tool in the game short of a Vindicare Surely you troll, the only thing Deathleaper is good at is giving -1 Leadership to a character of their choice and costing you a lot of points when it dies to a hail of gunfire), and Lictors (a unit that... okay, nobody likes Lictors, but at least they aren't this).
Additionally, they have one of the most schizophrenic loadouts in the game. Their attacks ignore all armor saves, which would be invaluable in 6th edition... except the rest of their stats make them absolutely awful in close combat, including I1, WS3, and only one attack. They're too slow to keep up with an assault-based army (and largely pointless there anyway), and in a shooting army they compete with Hive Guard (which are basically the only way 'Nids can take down tanks transports - tanks are dealt with via either Zoanthrope's Warp Lance, Abjuration Mechanicum psychic power, or Vector Strikes from overhead Flyrants/Harpies. Or the odd lucky Carnifex/Trygon that actually catches a tank too stupid to move). Their Flamespurt ability is AP 4, meaning you can't take out MEQ with it. They can explode, which could see them used as a suicide unit... except that affects your (pitifully weak, vulnerable to AoE explosions) units as well. Plus it's only when Instant Death is inflicted, which means melta or plasma... but it dies much easier to massed bolter shots.
So, all in all, the worst fucking unit the game. Completely pointless, no idea where it wants to be, competes with good units (units that are basically compulsory if you want Tyranids to win).
Fucking Cruddace.
Countless ideas are thrown about to unfuck the unit, but the cocksucking piece of shit is fundamentally contrary to the basis of the entire army. To make it fit homebrewers have pretty much had to make the pile of fail unrecognizable from it's origins. It is that bad.
Saving Grace
There is one saving grace to all of this. They look almost identical to Biovores (a unit that is a cheap source of assault pie plates that can also be used as a barrage, and has one of the derpiest faces around), but they have a much newer, much less derpy model (although their gun does look strangely like an erect throbbing penis). As such, pretty much everybody buys the Pyrovore models and uses them as Biovores. But this has the possible side effect of convincing GW that the Pyrovore is actually good...