Well, imo, Dusknoir is held back in that its typing doesn't grant it useful resistances. Other walls like Ferrothorn, for example, resist Rock, Dark, Ghost, Water, and others. That said, it has nice dual STABs and a powerful Focus Punch, but since it is hard to keep other weather off the field, it can really only truly shine on a dedicated Sun Team imo, which would still fair rather poorly against other weather teams.
Anyways, I am going the bring foward the concept of Suicune being OP. Now this is a very high argued topic, so I am going to put forth some reasons why it is OP. First off, I want to make clear what being OP is. Whenever I bring up this subject, the first thing I hear is something like "It's not OP, Magnezone, Rotom-W, Ferrothorn, and others can counter it." Just because something can be countered does not mean it is not OP. Deoxys-D, for example, was finally banned, yet Espeon, Scizor, Tyranitar, and others easily beat it. It was banned because it filled its role too effectively and reliably.That is really what OP means. In the case of sweepers, they typically need to be counterable to be OP. But when it comes to stallers/supporters, being OP means that they reliably wall most of the metagame too well or support their team too effectively. Moving this concept on to Suicune, we can see where problems are existing. Right off the bat, Suicune was buffed immensely by DM Universal changes. The disappearance of crits allowed Suicune to set up CM more reliable and effectively, and more importantly, access to the prized Water Absorb (later becoming Storm Drain) ability allowed it to wall things like Politoed, only fearing Toxic, and most Suicune at that point ran Rest anyway. Imo, this was where Suicune should have stayed, very effective, useful, but held back enough by lack of reliable recovery and support ability. However, we pushed it over the line when it got Wish. Wish is tbh the reason Suicune is OP. It gives Suicune two things that make it very powerful, reliable recovery and reliable team support. Suddenly, Suicune is more or less way better than other Bulky Waters, only rivaled by Milotic, and supports its team and itself with recovery. In addition, it can put Wish together with CM for a hard to beat offensive attacker. Suicune was clearly a great choice in OU without Wish, so why did we give it such a thing? It was probably because we did not think Suicune was viable yet, but tbh, it was. Now we see people forced to run some sort of Suicune counter in order to guarentee solid team success. Toxic is also a common method used to overcome it, but along with cleric support, such an option with not work very effectively. I would like to finish off this post by pointing out that "DM aims to balance the OU metagame". In a balanced OU metagame, we should not be forced to run counters to something, it should be a threat, but not a threat the metagame revolves around. My thoughts are that Suicune is good enough without Wish and that should likely be dropped to return some more balance to the metagame.
Oh, and I don't want to hear dumb comments like "Ferrothorn counters it", because I mentioned why that is not a good statement. :s