these calcs are irrelevant since all the attacks you're listing have drawbacks (are we really comparing draco meteor to pixilate hyper voice ?) while hyper voice has no opportunity cost
I also disagree on this:
A plethora ? Every wallbreaker in UU that isn't sylveon has a solid counter. And please don't say iron tail hydreigon or something, that's literally what lures are for
For the rest, you're basically saying that in order to win against sylveon, you have to constantly make the right predictions and double switch so that it doesn't have the time to click hyper voice. Sorry but that works against every mon and cannot be used as an argument to determine if a mon is broken or not
If mons was just calcs, then yes, Hyper Voice has no drawbacks. In practice, the second you fire off a Hyper Voice you risk inviting something like Nido/Entei/Roserade/literally any Fairy resist that can take a hit and immediately pressure your team. If you get a kill, great! Now your opponent can freely send out literally any wallbreaker ever to revenge it.
The -2 from Draco doesn't really matter when you obliterate pretty much anything with it minus Fairies which are all bopped by Specs Flash Cannon (yes it is a set, just because you don't see it everywhere does not mean it's bad). You might say something can take advantage of a -2 Hydrei after it kills something, and you'd be right. Similarly, there are so many ways to punish Sylveon if it ever clicks Hyper Voice or even worse, HP Fire or Psyshock.
Close Combat's drops, again not looking in a vacuum, are irrelevant in Luc's case since it gets the best priority move in the game making it incredibly difficult to revenge at +2. So how does one stop Lucario? Don't let it set up. How do you stop Sylveon? Don't let it get multiple free switches.
(Also Luc definitely has no counters, with its wide movepool and threat to NP and SD it can beat anything).
Sylveon has counters, c'mon now don't exaggerate. Blissey, Arcanine, Bronzong, Metagross, M-Aggron eat Hyper Voices easily. And please don't say Shadow Ball or something, that's what lures are for (also you'd be locked into Shadow Ball which is set up fodder for pretty much anything).
And no, I did not say you have to predict every turn correctly and constantly make doubles to win vs Sylveon. If you read, I was talking about it's matchup vs offensive teams where it gets (or should get) at most one switch in, and if the opponent takes it away, then your Sylveon is a liability. I even admitted even if he predicted wrong, I get to fire off a whopping one Hyper Voice before Entei/Nido come in and proceed to wallbreak (his team has ways to punish Sylveon, like a good team should).
Note: To avoid anyone believing I think Sylveon isn't broken because you can revenge it after it kills something, I wanna clarify that this is not what I think. There are plenty of offensive Fairy resists that can take a Hyper Voice and force it out due to its low speed and physical bulk. However, even if it gets a kill, it invites many wallbreakers in or is easily set up on if it is not locked into Hyper Voice.
Edit:
Machiavelli Except Sylveon invites in 99% of UU's wallbreakers, not just Nidoking and Entei, as a revenge kill so good luck accounting for that when you build.