Pros of Hydreigon: Hydreigon has an interesting defensive presence for a wallbreaker, boasting 92 / 90 / 90 defenses, Levitate to brag Ground and Spikes immunities, and nice Dark, Ghost, Fire, Water, and Electric resistances with a Psychic immunity, giving it some decent opportunities to check or even switch into some things. While there are greater mixed wallbreakers out there in terms of power, 105 / 125 mixed offenses are still impressive, especially with great moves like Draco Meteor, Fire Blast, Flash Cannon, Superpower, and Earth Power to abuse, and base 98 Speed lets it outrun plenty of walls and slow attackers (not a huge upgrade from Kyurem-B's base 95 Speed, but it's still notable). All of this means Hydreigon is very hard to switch into, as Fairy- and Ice-types don't want to take a Flash Cannon, Steel-types don't like Fire Blast and/or Earth Power, Chansey hates Superpower, and most everything else dislikes taking a STAB Draco Meteor. The Choice Scarf set isn't all that awful either, as it acts as a great revenge killer after Fairies are gone, working well against many offensive Pokemon, and U-turn is a neat little bonus that can net you an advantageous matchup when you predict a switch.
Cons of Hydreigon: Fairy-types are everywhere, Which means two things: 1) Hydreigon's Draco Meteors could wind up being duds, and 2) Hydreigon's 4x weakness to Fairy-type moves becomes rather glaring. This means Hydreigon is potentially a momentum killer as long as the opponent still has a Fairy-type on deck. Hydreigon is able to 2HKO many Pokemon, but is incapable of OHKOing most Pokemon, especially Fairy-types that can OHKO it, like Clefable, Mega Altaria, Mega Gardevoir, and Azumarill. This especially applies to the Choice Scarf set due to its move-lock effect, which is why it's such a risky and hard to use set in this metagame. When lacking a Choice Scarf, there are also many faster Pokemon that can take it out, like Mega Charizard X, Keldeo, Mega Lopunny, Latios, Mega Gardevoir, Weavile, regular and Mega Alakazam (Focus Blast is risky though), Mega Gallade, Mega Medicham, Scolipede, and Terrakion, further adding to its momentum killing flaw.
All honesty, I'm on board with Hydreigon dropping to B, as the Fairy-type issue is a big enough reason on its own imo, but being such exploitable revenge kill bait is just not something I can overlook. Drop Hydreigon to B Rank.
Man I hate typing on my phone. Sorry for any typos.