Hoopa-U seems to be a totally overpowered abomination. 680 BST, offensive stats nearly rivaling that of Mega-Rayquaza. A solid choice of moves, access to boosting moves too. Low speed to take advantage of Trick-Room. Access to a handful of high base power moves. Magician isn't a bad ability either, and it only has one weakness, to a lesser-used type.
I get that Hoopa-U is in Ubers (I think) but at this point it feels like something everyone would want on every team.
To quote's Hoopa-U's Gen 6 and Gen 7 Smogon analyses, the reason is this: Gen 6 Ubers analysis
Despite that, however, its viability should not be overestimated, as stall is the least solid playstyle in Ubers and Hoopa-U is vulnerable to all forms of status and entry hazards. In addition, Hoopa-U is hindered by its mediocre Speed and pathetic Defense, which leaves it susceptible to physical attackers and priority moves.
Gen 7 OU analysis
On the flipside, its middling Speed stat, despite being good for a wallbreaker of its caliber, leaves it at the mercy of a number of faster offensive threats in the metagame such as Mega Medicham, Kartana, and Tapu Lele. It also carries poor physical bulk along with a 4x weakness to one of the strongest momentum-gaining moves in the game, U-turn, meaning that Hoopa-U tends to get overwhelmed by the likes of U-turn Tornadus-T, Greninja, and Tapu Koko.
In short, the reasons are as follows: 1. 4x weakness to U-Turn is a severe downside, means it gets revenge killed by a lot of offensive threats. 2. Mediocre speed and pathetic defense means it easily gets revenge killed by faster offensive threats. 3. In the case of Gen 6 Ubers, stall-breakers not being in very high demand. Source: https://www.smogon.com/dex/xy/pokemon/hoopa-unbound/ https://www.smogon.com/dex/sm/pokemon/hoopa-unbound/
