Defensively dark is only resisted by ONE type and is easy to overwhelm especially with future sight support (since dark types love coming in)
While I agree with your post overall, it is worth noting that dark is not resisted by only one type.
It's resisted by 3; Dark itself, Fighting and Fairy.
Dark and Water both have the same amount of resists. Your point about future sight support is absolutely valid though, and it's one of the reasons I think Urshifu-SS is worthy of a suspect.
Speaking of suspects, I hate to add to suspect talk in the metagame discussion thread (there's enough of it as is), but I do think that there are some problematic elements in the meta at the moment. These are made more glaring by my belief that the metagame is actually in a pretty good place now. That is... outside of these elements.
1) Urshifu-SS
Probably my pick for the most broken mon currently in the tier. For reasons already mentioned in the thread (Wicked Blow, Future Sight support, excellent coverage), I do agree that Urshifu-SS should potentially be looked at.
I have been using this mon a lot recently, specifically band w/ Sucker Punch. It always catches me a little off guard how well this mon can do with even 1 correct prediction. Not that you even necessarily need to do that, you can use a pivot to bring it in with relative ease and kill a mon pretty regularly. Buzzwole is the one true counter, and it is checked by numerous mons. Even so, I feel this thing is too strong or OU.
2) Magearna
Agreeing with Omari P, this thing is still broken, as it was back in IoA meta. There is more counterplay than there was previously, I will admit. However, I think the additions it got this gen still just push it over the edge. Worthy of a suspect, perhaps even before Urshifu-SS. Specs and Shift Gear + CM are my least favorite sets to run into on the ladder.
3) Spectrier
The horse is, in my opinion, an uncompetitive Pokemon. In a similar vein to Zygarde last gen, it both excels with just one move, and has numerous viable sets to overcome it's counterplay. I have been running a lot of scarf recently, and finding great success. It's an amazing cleaner, akin to a Scarf Vish. However, if I ever got sick of Blissey, I could very easily run Sub + Disable. If I ever got sick of sucker punch, Sub + NP. It's gross, and I feel bad even using this mon. Specs and Scarf, which are its best sets (I think), do not reward skillful play. They reward mindlessly pivoting into this monster, where a good matchup is a winning matchup. It requires running a Sp. Def Dark type, or a Blissey on most teams, generally speaking. I'm not a fan and would like it out of OU. I do admit that this one is the shakiest of my three picks for a suspect. I hold that it should be at least looked at, as I'm sure the council is doing.
--------
That being said, we did just have a suspect on Pheromosa. So I understand if the council wants to take some time to assess the metagame and let it develop. I'm only a mid-ladder player after all. I have faith that they will be open about what is happening, and consider what people are saying in the community, as they have clearly done to this point in gen 8.