I think Poison is the only type that's clearly disadvantaged overall, really.
Most of the rest I think, with no offense meant to anyone, is simply players who don't understand the types and how to use them effectively, or who have had bad experiences with one or another of the poor examples of that type and have extrapolated that out to a judgment of the type as a whole.
And a LOT of the problem people have with types goes back to a basic failure to understand in-game strategy - something that I see constantly on in-game RMT threads. Point number one - ALL types have a disadvantage against one or some other types. If you leave a type in against something that's SE against it and it gets hit and faints, that doesn't mean the type is bad - it means you used bad strategy and kept the wrong pokemon in. Switching might be a problem in competitive battling, but in-game, it's effortless and virtually risk-free. I can't even count how many times I've seen someone say, for instance, that Grass types are disadvantaged because they don't get attacks that are SE against Flying types. Say what? All that means is that you shouldn't leave your Grass type in against a Flying type, which with the fact that their STAB attack is going to do SE damage anyway, would seem pretty obvious.
Poisons though are disadvantaged, since they only have one SE attack, against Grasses, and it's not as if being able to hit Grasses SE is anything special anyway. Even with that, they could be okay - after all, Dragons only have one type they're SE against too. But Poisons have two weaknesses, and that's Ground and Psychic - among the most common physical attacking type AND most common special attacking type, and some of the bulkiest (or fastest, in the case of Psychics) among them to boot. Dragons, on the other hand, also have two weaknesses, but that's to Ice - one of the most fragile types out there - and other Dragons, which effectively cancel each other out. So Poisons are left without a good niche - they only hit hard against a type that's relatively weak to begin with, and take hard hits from two of the most powerful types in games. The only thing they have left is a fairly broad range of neutral STAB, but they don't have any powerful attacks to take advantage of that. They don't even have their little niche of being able to poison any more, since everything in the game can learn Toxic.
They need more bulk, to better withstand the powerful attacks they're weak to. It's not a coincidence that Muk and Weezing were pretty good back before the physical/special split, when it was easy to predict what type of attacks an opponent was going to use and safely put one or the other in. But with their heavily skewed defensive stats and the relative unpredictability of attacks since the split, it's much harder to depend on a pokemon who's only bulky on one side and is notably weak on the other, since it's much easier for an opponent to catch you off guard with an attack on the weak defense. If they had good bulk in both defenses, they'd last better. And they need a few much more powerful attacks - base 150 at least - to take advantage of neutral STAB. I think that would go a long way toward fixing them up, and without altering the type chart at all.
