Sorry for the late reply (I was really busy the night before so I had no time to reply). Anyways,
I think what makes Registeel rather mediocre in UU is the fact that it receives really stiff competition from other walls. It can work, but Bronzong, Cobalion, and Empoleon give it really tough competition for a Steel-type wall that can set up SR, as they have arguably better defensive typings. Registeel is also rather weak, so it tends to be a momentum drain a lot of the time. It also as a result attracts some really dangerous Pokemon such as Mienshao and Victini into the field. That said, it can work as a Raikou and Roserade counter and such, but it's often outclassed.
As for the second question, Misdreavus is better than Dusknoir because it has nearly the same bulk with Eviolite factored in, is immune to Spikes, and has stronger STAB to work with. Misdreavus can use Shadow Ball, which is relatively strong, but Dusknoir has to resort to the weak Shadow Punch for STAB. It does not help that Dusknoir lacks many powerful moves, with many of them having 75 BP, which means anything that takes neutral damage can take it on rather well. Misdreavus also has Nasty Plot to work with offensive sets, as well as Heal Bell to be a solid team player. Dusknoir can be used if beating both Kabu and Cryo as well as Leftovers recovery is desired, but overall Misdreavus is often better because it hits harder and also has greater utility.
