Hoopa is not bad, in all honesty. What priority besides Sucker Punch are super common that Hoopa isn't resistant / immune to.
Refrige Kyu-B and Pixel Xern both say hello.
Anyway, now that I have a moment to give my viewpoint.
If we add an ability clause, a lot of teams are going to be hurt, both stall and offense. Stall can't run multiple bouncers and the like, offense can't run multiple Contraries and Adapts, and neither can run multiple PHers.
This is fine. I mean, playstyles get hurt, but they also get buffed. For example, stall no longer gets shut down by Regenerator-heavy teams that just shrug off residual damage for free. Stall also no longer has to worry about making a mistake against a Contrary-heavy team and getting swept. Meanwhile, Offense isn't getting Prankstered to hell while trying to find some momentum or trying to figure out how to even dent a group of well synergized Poison Healers.
Hyper offense and hyper stall probably get hurt the most by this. Balanced probably benefits the most.
Also, this situation is really comparable to species clause in standard. For example, would you say it's healthy for OU to be able to run a team of six Gale Wings Talonflames? No, probably not. What about six PH Gliscors? Unless you're packing ice moves, heck no. What about six Serene Grace Skymins in Ubers? I can see you grabbing your pitchforks already.
So why is it considered healthy to be able run that, and worse, in BH?
Also mind, I'm in favor of an ability clause despite the fact that I won't get away unscathed by it. I think I have just one team that wouldn't need updating. The rest have dual PHers, dual Regens, dual Flashfires, dual Adapt,
triple MG, and more.
And finally, to bring back the replay I posted, it avoids silliness like I did there. Kumi had a great counter to PH users, specifically that MB Skarm with Simple Beam. But that one Pokemon wasn't able to handle a whole team of PH users on its own. Likewise, it'll typically be impossible for a single counter to stop a whole team abusing a single ability that's built and played by someone competent.
Single-ability clause, double-ability clause, whatever. All I know is that current status quo is bad.