In my experience, Chi-Yu is far from being broken, unless we take Terastal in mind, which at the moment is dealt with by a Suspect. With Terastal it is broken due to being able to abuse both a Super Stab and Terablast (its one of the only Mons that can use that move very effectively) to destroy some check (Grass for Azu, Fight for Tar, Fairy for Roaring Moon).
So, I will provide checks and counterplay to Chi-Yu without taking Terastal in mind, since this is a mechanic that has a big variance in each match and makes many Mons broken and uncountearable, not just Chi-Yu. Of course, the fish has many Sets and universal checks are few:
-Tyranitar. Obvious one. Gets ruined by WoW, but even burned can deal with the fish easily.
-Roaring Moon. Lives both STABs. Can run bulky DD + Roost Sets pretty well, has EQ to OHKO fish. Doesn,t like burns (which not every Chi-Yu uses), but due to DD, its not the end of the world.
-Azumarill. Not an actual check, however if Chi-Yu is choiced and kills something, can abuse it by using Belly Drum.
-Ting-Lu. Only takes relevant damage from Specs Overheat, totally ignores other sets. Restalk Ting-Lu is viable, just run another Hazard setter along it and you are fine vs Specs too.
-Quaquaval. Checks Scarf, has Roost. Obviously can,t switch into Specs, but Scarf is more common.
-Paldean Tauros, Fire or Water. Same like the ugly duck.
-Dragonite. Due to Multiscale, can switch into every Move at least to scout. Can run defensive sets.
-Hydreigon. Switches once into Scarf. In theory could run a Restalk set, though its hard to differentiate from bulky Roaring Moon sets.
-Gastrodon and Clodsire. Have big Sdef and Recover. Can fail vs Specs, but Scarf can,t break them.
-Blissey and Chansey. Only NP or Specs + Sun can get past them. The NP ones need Taunt, since otherwise they get a Thunder Wave.
-Rain. The fish might get to kill something, but this playstyle in general gives him big troubles.
If using Hippowdon (preferable with Smooth Rock):
-Garganacl. Due to the Sand Sdef boost, laughs at everything Chi-Yu can do.
-Klawf. With Assault Vest, Sand boost and Regenerator, doesn,t care about anything and can use Knock Off, similarly to what Tornadus-T did in ORAS. Its a bad Mon, but decent enough in Sand.
-Coallossal. This one has to use Boots + Restalk to be viable in Sand, but it gets Spikes.
-Glimmora. Like Coallossal, needs Restalk, but could succesfully run Corrosion with it (except vs Gholdengo).
Some other unranked Mons:
-Hariyama. With Restalk + Thick Fat, it only fears Terastal and Psychic. Can spread Paras with Force Palm and use Knock Off.
-Dachsbun. Didn,t try this Mon, it looks bad. However, it fully counters Chi-Yu by being immune to fire and resisting Dark.
I get that many of these Mons are not easy to fit and some are only situational checks or depend on the set. However, actual games are different from theory and in my experience, there is enough counterplay to Chi-Yu. It can use Terastal, but so can do the opponent. Depending on how the Suspect ends, it might end up broken, but right now I only think its broken with the Terastal mechanic, and the same thing happens to many Mons, so I don,t think Chi-Yu is that much superior, even though its undeniably one of the best OU Mons. Annihilape, Cyclizar and Gholdengo (in this specific order) are the only Mons I think are actually broken regardless of Terastal.
