I don't think anybody is denying that Hoopa-U doesn't have a solid switch in. Let's look at the options to deal with it:
- Offensive Pressure
- Limit switch in opportunities
- Pursuit trap
If you're using full stall, we can safely eliminate the first two, because that's not really how stall works. I think the comparison to other breakers is pretty useful, I'll explain why. Let's just assume that you do in fact prepare for Hoopa-U when using stall, and run that pursuit trapper. I realise it limits building (but crucially does not make the playstyle unviable), I'll get on to that later. Hoopa-U now has the major disadvantage, if the stall player can manage to get about 30% off on the 80/60 physical defence mon with no resistances, of being unable to lock itself into any move other than focus blast, else it risks getting trapped and killed.
This limits Hoopa-U to one kill. How many other breakers can you do that for? Manaphy is very difficult to counter too, due to its coverage and Tail Glow set up abilities, so stall players usually turn to sp def unaware clef, or just hope that it doesn't run a certain coverage move. Now, look at what unaware clef actually does. It switches in a lot, and will get scald burned and worn down quite quickly. This means, that if the Manaphy user can figure out a way to wear down the opposing Clefable, despite the stall player running a decent counter for it, then they can still win with Manaphy. Now switch out Manaphy for Hoopa-U, and Unaware Clefable for Tyranitar. Hoopa-U can't still win, once it gets one kill, and then gets pursuit trapped, because it can't escape from that. Being able to go one for one on what is almost certainly the opponent's best breaker, and more than that, being able to choose what you sack seems like a pretty decent deal to me, once you've got the small amount of damage, and have prepared for it in the first place.
I'm not going to deny that Hoopa-U is restrictive, and it does force the meta towards a more offensive direction. However, all styles have counterplay to it, with varying degrees of restrictiveness. This isn't a gamebreaking threat that absolutely forces you to run 6 hyper offensive mons, or lose.
Also, importantly, Hoopa-U does not generate its own switch in opportunities. It's not as simple as coming in and getting a kill every time, mostly because of how difficult it is to come in safely in the first place, it doesn't have any useful resistances, and Hoopa's physical bulk is extremely lacking. There are certainly ways to adapt to Hoopa, and this might involve making a balanced team a little more offensive than it would otherwise be, and running a specific pokemon from a list of pursuit users on stall, and there are other pokemon such as Manaphy or Gardevoir that are also very restricting in this sense.
The fact that there is the option to adapt, that this does not involve running otherwise unviable mons, and that the metagame still remains competitive, albeit admittedly more offensive, with Hoopa-U makes me believe that it is fine to stay in the tier.
This is just pointing out what can be done, that adaptations do exist, and the meta does have a way forward with Hoopa-U. It is certainly true that bulky builds are much more free without Hoopa. I think that to ban it due to that reasoning, would be based on personal preference. As it is, Chansey + Skarmory + Sableye builds have proven to be a very safe way to ladder for a good number of players, and other very bulky builds are effective too. This is as a direct effect of removing an extremely effective non-mega breaker from the tier. I've tested out a couple of teams on the ladder now, one of which was stall. I don't find stall dittos very entertaining at all, nor laddering with stall to be a particularly enjoyable process. In the end I've decided to use what I prefer and ladder with a team really packed with breakers, specifically built from the ground up to give stall an awful matchup. This is pretty important, because anyone who ladders on this suspect a lot will find that stall builds are very common, and that you have to gear your teambuilding process much more towards breaking stall than ever before. That being said, this ladder just simply isn't as enjoyable as the non suspect ladder for me.
Keep in mind that this last paragraph is not an objective argument. If anybody wants Hoopa-U banned specifically to make very bulky, passive builds more viable, then I believe this is showing personal preference for the meta that you want. As for that, well, we're simply never going to agree. Comparing the suspect ladder, and the old non suspect ladder, because the ladder should be an important factor in any decision about tiering policy, I find the non suspect ladder to be much more enjoyable. Based on my own personal preference, a metagame with Hoopa-U is much more enjoyable than one without it.