So, I've been looking at these posts and thinking since my last post, and have come to the general consensus that it should probably be banned. As much as stall makes me want to rip my own hair out and eat it and subsequently down a glass of nitric acid to wash the hair down, I have to accept it's just as viable a playstyle as offense, or at least, it should be, like the endangered tarantulas of the Pokemon world - Not many people like them, but they're still endangered and should be protected. Why? It's too easy to support. Slap on a few pivots, some volt-turn and it's virtually guaranteed to get at least a kill a match, at worst MASSIVELY chunking a switchin. Hell, this thing can be your main offensive option on a team and it can still do work. I've seen matches where it kills FIVE POKEMON. FIVE. U-turn coming in? Just switch out to Lando and U-turn the next turn! Talonflame? Easy, just go into Rotom! It's kind of stupid that it only really needs two teammates to be at max efficiency, and I should know, I've abused this. Hell, sometimes it doesn't even NEED support! If the opponent Dracos with their Lati, or kills something with Thundurus-I, just go straight into Hoopa the next turn and get a 1for1 at worst! In some matchups, it turns those otherwise fantastic special attackers into liabilities, because the offensive teams you find them on are usually so frail that Hoopa will almost always KO something.
I built a stall team for giggles and found that, in order to not autolose to Hoopa, I HAD to run a ScarfTar, a Pokemon that doesn't really fit on stall, turning it from stall into semi-stall and being just generally constraining on teambuilding for defensive playstyles. It's not overcentralising for offense, but for defensive teams, it's almost as bad as damn Mega Kanga - Forces you to run a specific check to not autolose to it. Emphasis on CHECK (remember how +2 Crunch from MKanga could OHKO Sableye after rocks?).
Overall, I think it should be banned as it works well - too well - against all playstyles with minimal support (potentially) and is virtually guaranteed a kill, at worst hugely contributing to a kill later in the match by another sweeper, who that wall would otherwise take on. Stall isn't the only playstyle that suffers from Hoopa's presence, and as much as I love it's design, that kind of bias shouldn't stop me from recognising a clearly overpowering Pokemon.