The main thing to keep in mind about Zygarde's Complete Forme is that without a Focus Sash, the odds of getting it to appear aren't very good when you have Ice-type attacks trying to wipe out the 10% or 50% formes. And even then, entry hazards like Stealth Rock can render the Focus Sash useless.
Not true - it has a few other options. It could run a yache berry or substitute. But even if it doesn't, 50% Zygarde is extremely bulky in its own right. Here's some common ice-type attacks in OU.
252 SpA Life Orb Tapu Koko Hidden Power Ice vs. 200 HP / 252+ SpD Zygarde: 203-244 (49.8 - 59.9%) -- 80.9% chance to 2HKO after Leftovers recovery
252 SpA Genesect Ice Beam vs. 200 HP / 252+ SpD Zygarde: 276-328 (67.8 - 80.5%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Manectric-Mega Hidden Power Ice vs. 4 HP / 0 SpD Zygarde: 284-336 (79.3 - 93.8%) -- guaranteed 2HKO after Leftovers recovery
Weavile and Black-Kyurem are both capable of OHKOing that set, but do remember, they have to swap in first. Pheromosa can as well, but depending on the EV spread, it might be a gamble. And it'd really dislike a +1 extreme speed from a more offensive set.
I'm using a gen 6 set for the calcs, btw, since there aren't any standard smogon sets for gen 7 yet.
0 Atk Zygarde Thousand Arrows vs. 0 HP / 4 Def Weavile: 136-162 (48.3 - 57.6%) -- 94.1% chance to 2HKO
0 Atk Zygarde Extreme Speed vs. 0 HP / 4 Def Weavile: 81-96 (28.8 - 34.1%) -- 2.9% chance to 3HKO
Weavile swaps in, breaks the sub, then loses a minimum of 48% of it's health. 58%. Then extreme speed chips off another 29%. 87% of you health. Then life orb saps another 10% - best case scenario, you're stuck with 3% of your health left. That's only if damage fluctuations favor you on the extreme. More likely, you'll either fail to bring Zygarde down or you'll lose your Weavile in the process. It's worse for the likes of Mega Manetric and Tapu Koko - if they swap in on it while it uses a sub, they've already lost.
It's actually incredibly, incredibly easy for Zygarde to find a chance to set up. A random ice move won't always be enough to stop it. Far more then most sweepers, in fact. I've been on the ladder for a good while now, and I've already been screwed over by this thing in a number of different ways. He's not exactly Power Herb Xerneas or Mega Rayquaza, but I'm still 100% confident this guys going to Ubers regardless. He's certainly worse then the likes of Aegislash and Greninja, imo.
On a side note, Zygarde's options of Dragon-type physical attacks are very limited, since it can only go with Outrage or Dragon Tail. And obviously, Outrage is not the best kind of attack to be using, while Dragon Tail has decreased priority and isn't all that strong.
Zygarde doesn't care that much though, because Thousand Arrows is just so good. Only grass types and the rare bug resist it.