Okay, let's see the water types I meant:
Normal Slowbro
+1 252+ Atk Life Orb Zygarde Outrage vs. 252 HP / 232+ Def Slowbro: 239-282 (60.6 - 71.5%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
0 SpA Slowbro Ice Beam vs. 4 HP / 0 SpD Zygarde: 272-320 (75.9 - 89.3%) -- guaranteed 2HKO
with rocks and LO recoil, you can get KOed, and he somehow does not, he can pivot to a fairy to regenerate damage and possibly capitalize the confusion.
Suicune:
+1 252+ Atk Life Orb Zygarde Outrage vs. 252 HP / 252+ Def Suicune: 230-270 (56.9 - 66.8%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Can kill you with some prior damage, burn you, rest stall the confusion, or phaze you out depending on your tastes.
I was not counting Rotom, who lacks ice moves barring HPst Ice, but anyway he has a very good chance to survive and burn you.
252+ Atk Life Orb Zygarde Outrage vs. 252 HP / 252+ Def Quagsire: 187-222 (47.4 - 56.3%) -- 84.8% chance to 2HKO after Stealth Rock and Leftovers recovery
Okay, I can give you that one. He with rocks he can get stomped, tough locking yourelf in Outrage always leads to trouble with Cleff around.
Saying that Talon and Weavile go down after rocks is like saying Shuckle goes down after Taunt lol. Thanks Captain Obvious. I was mostly thinking on the lati twins, Jirachi, and Ice Shard users like Mamoswine that only requires a little prior damage (read, Spikes) to kill:
252+ Atk Life Orb Mamoswine Ice Shard vs. 4 HP / 0 Def Zygarde: 317-380 (88.5 - 106.1%) -- 43.8% chance to OHKO
Compared power:
+1 252 Atk Tough Claws Mega Charizard X Flare Blitz vs. 0 HP / 0 Def Mew: 382-450 (112 - 131.9%) -- guaranteed OHKO
+1 252+ Atk Life Orb Zygarde Outrage vs. 0 HP / 0 Def Mew: 351-413 (102.9 - 121.1%) -- guaranteed OHKO
+1 252+ Atk Pixilate Mega Altaria Return vs. 0 HP / 0 Def Mew: 318-375 (93.2 - 109.9%) -- guaranteed OHKO after Stealth Rock
+1 252+ Atk Life Orb Zygarde Earthquake vs. 0 HP / 0 Def Mew: 292-344 (85.6 - 100.8%) -- 81.3% chance to OHKO after Stealth Rock
+1 252 Atk Mega Tyranitar Stone Edge vs. 0 HP / 0 Def Mew: 291-343 (85.3 - 100.5%) -- 81.3% chance to OHKO after Stealth Rock
+1 252 Atk Mold Breaker Mega Gyarados Waterfall vs. 0 HP / 0 Def Mew: 223-264 (65.3 - 77.4%) -- guaranteed 2HKO after Stealth Rock
As said, Charisuck wins, and Altaria, the other DD user I meant, is not far behind either. Plus from the two you mentioned, only Gyarados has a notable difference in damage, and that's discounting that none of those rely on a multi-turn move that can be easily taken advantage of with a proper steel-type or worse, a fairy type. There comes the difference: WHEN USING EQ as the spam attack, Zygarde is much weaker. When using Outrage, he defeats Altaria, at the cost of getting revenge killed instantly thereafter. Even those weak frail mons you mentioned can easily capitalize that.
And when I said "slow teams" I mean "teams lacking Ice Shard, Prankster Thunder Wave or Choice Band Talonflame with enough luck to get a timely crit", because with a good amonut of HP EVs not even rocks gives talon a chance to OHKO the dragon.
252 Atk Choice Band Talonflame Brave Bird vs. 216 HP / 0 Def Dragonite: 223-264 (59.1 - 70%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery.
Now that I got to use the calculator with calm, I wonder why the rotom formes lack any precoded set and why the DD Zygarde set has Stone Edge instead of Extremespeed...