Punchshroom
As far as I'm concerned, the biggest thing about the Pursuit argument with Hoopa-U is that it's now mandatory to run Pursuit on everything but heavy offense. By the very nature of the Choice Specs set alone having no counters, unless your team is so fast and offensive that Hoopa-U can barely do anything, you pretty much auto-lose to Hoopa-U unless you have Pursuit. I don't honestly think I can name a single other Pursuit-bait Pokemon that you auto-lose to without Pursuit. Even Pokemon like Hydreigon and Kyurem-B still have switch-ins depending on what moves they run (even if they, strictly speaking, have no counters overall) so you can scout for what they have and act accordingly; hell, Greninja couldn't claim to dismantle everything defensive with four or fewer moves and even that was considered to put enough pressure on balance to warrant a ban (but to be fair, its higher Speed, which Hydreigon and Kyurem-B lack, is what put it over the edge).
Also, I'd like to point out that 5/8 of the stronq wallbreakers you mentioned are Mega Evolutions and the other three have actual counters. By the very nature of Mega Evolution, there is an inherent opportunity cost to running those Pokemon, and, furthermore, they are restricted from running any kind of power-boosting item like Life Orb or Choice Specs. There is virtually no opportunity cost to running Hoopa-U on your team, and I can't honestly see "lack of defensive synergy" being one seeing as it has exceptional Special bulk for an offensively-oriented Pokemon. Think of it this way: Imagine if there were a Pokemon with comparable Special Attack to Mega Gardevoir, comparable Special Defense to Mega Gardevoir, more base HP than Mega Gardevoir, only marginally weaker Attack-wise than Kyurem-B, and wasn't a Mega Evolution and therefore allowed you to run another Mega and hold a power-boosting item like Life Orb or Choice Specs? Actually, you don't need to imagine it because it exists already. It's called Hoopa-U.
Furthermore, about the whole "it's not so good against offense" argument (even though there are offense threats like Alakazam that Hoopa-U comes in on pretty easily due to the aforementioned Special bulk, unless you wanna start running Signal Beam Alakazam or something): Landorus-I wasn't exactly sweeping offense either.
As far as I'm concerned, the biggest thing about the Pursuit argument with Hoopa-U is that it's now mandatory to run Pursuit on everything but heavy offense. By the very nature of the Choice Specs set alone having no counters, unless your team is so fast and offensive that Hoopa-U can barely do anything, you pretty much auto-lose to Hoopa-U unless you have Pursuit. I don't honestly think I can name a single other Pursuit-bait Pokemon that you auto-lose to without Pursuit. Even Pokemon like Hydreigon and Kyurem-B still have switch-ins depending on what moves they run (even if they, strictly speaking, have no counters overall) so you can scout for what they have and act accordingly; hell, Greninja couldn't claim to dismantle everything defensive with four or fewer moves and even that was considered to put enough pressure on balance to warrant a ban (but to be fair, its higher Speed, which Hydreigon and Kyurem-B lack, is what put it over the edge).
Also, I'd like to point out that 5/8 of the stronq wallbreakers you mentioned are Mega Evolutions and the other three have actual counters. By the very nature of Mega Evolution, there is an inherent opportunity cost to running those Pokemon, and, furthermore, they are restricted from running any kind of power-boosting item like Life Orb or Choice Specs. There is virtually no opportunity cost to running Hoopa-U on your team, and I can't honestly see "lack of defensive synergy" being one seeing as it has exceptional Special bulk for an offensively-oriented Pokemon. Think of it this way: Imagine if there were a Pokemon with comparable Special Attack to Mega Gardevoir, comparable Special Defense to Mega Gardevoir, more base HP than Mega Gardevoir, only marginally weaker Attack-wise than Kyurem-B, and wasn't a Mega Evolution and therefore allowed you to run another Mega and hold a power-boosting item like Life Orb or Choice Specs? Actually, you don't need to imagine it because it exists already. It's called Hoopa-U.
Furthermore, about the whole "it's not so good against offense" argument (even though there are offense threats like Alakazam that Hoopa-U comes in on pretty easily due to the aforementioned Special bulk, unless you wanna start running Signal Beam Alakazam or something): Landorus-I wasn't exactly sweeping offense either.