First off, Whimsicott isn't "completely irrelevant" when it has extremely viable sets in OU and there has lately been a rising usage of defensive Lagging Tail sets. Salamence's EVS can be modified and even with only 64 Attack EVs and LO, it has over 91% chance of 2HKOing Belly Drum Azumarill with Earthquake. Granted Togekiss might be a little uncommon, there aren't that many Fairies either that's truly viable in OU and it still does get usage.Sigh. Scarf Hydreigon does not run flash cannon, Life Orb set does, which is by far the better set. Non-AV azumarill is 2HKOed after rocks by modest LO earth power, togekiss is really uncommon, and Whimsicott is completely irrelevant.
If you really want to you can run Iron Tail to nab the 2HKO on the occasional Sylveon.
You're telling me that a stab 4x super effective move will kill a hydreigon? No shit.
You did get me there with the LO thing, been a while since I played BW OU where LO wallbreaking Hydreigon was much more common and most of the Hydreigons these days roaming UU are Scarfed, so that's what I assumed you meant.
As for the STAB 4x SE = OHKO -- "no shit" indeed, and is something you should really think about when considering Hydreigon to break fairies.