Poisons. The problems with them are myriad and stack on themselves.
1) The cost structure. They're just WAY too expensive, for how difficult they are to acquire. There is zero reason why a mid-high level character can't eventually just get a big old pit of giant serpents, toss in a few pigs every now and again, and pull some out to milk for venom (after knocking them out in whatever fashion is preferred) whenever they feel like it, having an arbitrarily large supply. They're just animals, with almost no CR and minimal Int, and milking a snake is a totally normal thing to do. But with a valuation of 150 GP, the ability to 'create' 1000+ GP of "value" at will becomes somewhat problematic. In turn though the high cost also precludes normal acquisition and use, as no one in their right mind is going to jump through many legal and societal hoops to buy a few doses of venom to apply to a weapon in lieu of say a magic item.
2) The crafting of them. In no small part due to the cost structure, crafting them is just silly. Taking WEEKS to craft a giant serpent venom poison when it takes 3 whole MINUTES to milk it off of a giant serpent, and that's the baseline one. With costs seriously ranging up to those of mid tier magic items, but crafting rates commensurate with wood working or something, it could take YEARS to craft one dose of poison, apply it to 3 arrows, and have a fairly substantial boost to damage output (if and only if you're fighting a creature not immune to poison) for a single round of combat.
3) The mechanics of application. You can apply them to a weapon or arrow as an action, and then it lasts for one minute. For a melee fighter, the balance at least makes some sense. For a ranged combatant, you're trading an action for one Round of buffs instead of one Minute of buffs. The wild disparity and asymmetry alone is problematic. But also the idea that it's losing efficacy, even if in a vacuum sealed airtight container where it's in contact with the weapon, after a minute, regardless of the composition of the weapon, is just silly. The idea you can't have a bone arrowhead as the stopper in a vial of poison and pull it out, pre-poisoned and ready to use, is ridiculous.
4) The damage mechanic and type itself. Poison damage tends to be Huge numbers, which theoretically justifies the huge price tag, but it's also resisted or negated entirely by a huge portion of high level opponents. Undead, constructs, demons, elementals, you name it. If it's not a person it's likely immune, and even if it is a person there's at least a reasonable chance they've got something for it. So if someone were to go to the effort of obtaining and using this stuff, there's a very reasonable chance it wouldn't actually do anything.
5) The flavor of it. Possibly the worst offender of all, Poisons by and large just aren't interesting or reasonably representative of what poisons do in reality. We all have a vision in our head of someone getting hit by a poisoned weapon and slowly weakening and dying without treatment over time. The mechanics of it in game though treat it just like acid or fire, albeit far easier to resist, outside of a few attempts in the general direction of doing something other than direct immediate damage. Nothing in the game though gives that feeling of round after round of persistent effects weakening someone from a single cut, until eventually they fall unless aid is received.
All of this combines to make it a massive headache for DMs and players alike, with little to no payoff on either side. It's enough to make someone want to make a world with no snakes, spiders, wyverns, or other venomous creatures in it, and just say 'poison doesn't exist', because in 5e as it stands, it's just not worth it. I don't think that's really a hot take either, I'd be very surprised if there was someone tremendously happy with how they're implemented in 5e.
(By the way, though I'm not even going to try to propose a total overhaul of the system here, I just want to point out it definitely Could be done. In general poisons should apply levels of exhaustion on a failed save and persist for quite a while, until X successful saves or X period of time or until treatment is received. More deadly poisons have higher DCs, shorter save intervals, and / or more levels of exhaustion received. You COULD make poisons dangerous, cool, and interesting within the framework of the rules of 5e. D&D 5e as it stands just doesn't do so.)