I think most pseudo-legendaries have the issue of being pretty bad in-game. Think of DNite: It evolves at Lv 55 which means it is stuck as a Dragonair for pretty much the whole game until the Elite 4. Hydreigon is even worse since it usually won't evolve until post-game. Good luck hitting something while you have the Hustle Ability. They also have pretty bad movepools outside of egg moves and TMs (which is a problem until Gen 5)
Slow pokemon are also pretty bad. The Slowbro family, Reuniculus , Conkeldurr, Musharna all pretty much have to eat a hit before killing an opponent.
However, Special mention must go to Ditto, Smeargle and the Chansey family.
Hustle is pretty bad when it misses, yes, but it's a 20% chance for you to miss, normally. Opponents using Sand-Attack lower your accuracy more. And Hustle comes with a pretty big boon, making Zweilous hit hard enough to take out large chunks of the E4. It's Dark typing is pretty godly in Unova's E4 and its Dragon typing helps it sponge hits from Jellicent and Chandelure. It's not completely useless. And then in BW2, the levels of everything increase, and it doesn't exactly take hours to grind to lv. 64 thanks to Audino and Lucky Egg, in addition to being caught at lv. 50 instead of 40 like in BW. In XY you can catch wild Hydreigon if you try hard enough, and even if you don't you can still catch and train a Zweilous. You should be lv. 64 by the League anyways.
Dragonite's main problem is that it is not available in the games with Lucky Eggs and Audinos. And infinite use TMs. If you've ever used one in a playthrough, you can see how good it is once it hits lv. 55.
Metang's also pretty good in BW2. You just pretty much can't catch it since its catch rate is 3. You can also find wild Metagross in shaking grass, which is pretty epic.
Conkeldurr and Musharna are amazing, at least in BW. Sure they have to take hits, but they can take lots of hits and dish out lots of damage, especially Conkeldurr. Or even Eviolite Gurdurr, which trades power for a bit more bulk and no trading requirement. Timburr comes with STAB in Low Kick and gets strong STAB in Wake-Up Slap which gets you through Lenora. Then you evolve, pick up Hammer Arm, teach a couple TMs, and you're all set. Musharna is awesome because of its early evolution into a really bulky and powerful attacker, and your only Psychic-type at that point other than Woobat. It does get worse later on but it's still really bulky. Reuniclus isn't too bad either, thanks to its power and Magic Guard.