People who say its defenses are overrated, think about it.
Jellicent was considered one of the premier special walls of BW OU. And guess what? It has a HP of 100 and a Special Defense of 105, and a defense of just 70! Yes, it had brilliant typing and Will-O-Wisp etc but the fact of the matter is, it has far lower raw stats than Zygarde.
Now I know you can't compare the two, but Jellicent is a good example of how raw stats don't determine viability.
Zygarde may have a low base stat total (Legendary standards lawl) but Coil and Dragon Dance definitely make up for it as well as incredible (unresisted?) STAB. So will it be OU? Yes, because it has a good enough attack (as good as Kingdra's, who is considered a beast) fantastic HP (higher than Jellicent, Ferrothorn and like a whole load of things) incredible defense and good special defense (it's not hopelessly bad) and even good enough speed to top it all off.
So yeah, it won't be Ubers, sure, but that doesn't make it Kyuremly bad.
Jellicent has a UNIQUE typing. Zygarde's typing is... Garchomp... Plus both of these Pokemon have negligible abilities (if anything, Garchomp's ability is better...). The problem for Zygarde is that while it's got "good stats", there's an almost identical Pokemon with basically BETTER stats.
Why are Garchomp's stats better?
first let's talk about offense. 130 ATK is great-- it's basically just good enough to do whatever you want with it; it can wall break, sweep, or revenge. If you have 130, no one is worried about your offensive abilities. 100 is a different story.
100 can be really good or really bad, but it's completely dependent on all the factors of the metagame. Starmie has 100 Sp.A, but 100 Sp.A is incredibly good on a special attacking water type-- Ice Beam + Surf/Hydro Pump, or Thunderbolt + Surf/Hydro Pump is powerful, reliable, and most importantly, VERY difficult for major threats in the metagame to handle. 100-105 is a REALLY good special attacking stat for an offensive Water-type like Starmie, Rotom-W, or Greninja. If you're like Victini and have a 180 base power STAB move that can be boosted in sun-- well, yeah then, 100 ATK is more than enough. Also, when you have DOUBLE speed in Rain like Kingdra (which is huge), plus you can spam those same powerful special Water attacks, a lower special attacking stat isn't a big deal-- especially when you have a RAIN boost.
Zygarde is in a totally different boat, and we KNOW this because of another Dragon-Ground relative-- Flygon, who also has 100 ATK. Through the last 2 gens, Flygon has proved itself to be "just too weak" in almost every metagame it's been in (despite having Outrage and EQ). It is weak to the point that "....as disappointing as a Flygon's Outrage" actually became a common saying in our community 4th gen. In 4th gen OU and 5th gen UU, Flygon was a good poke because of its good typing and resilience to hazards, but in both metas (yes, even in last gen's
UU)-- that 100 ATK always held it back from being a truly top-tier threat. 100 ATK is not good enough for Dragon/Ground coverage-- even with +1 boosts from Coil or DD.
The big difference between Starmie and Flygon (and therefore Starmie and Zygarde), despite having the same 100 base offensive stat, is simply this-- breakdown of the metagame. Surf / Ice Beam / Thunderbolt coming off 100 SpA is a lot scarier than Outrage/EQ coming off 100 ATK just because the former is a lot harder to deal with using OU threats. Outrage coming off 100 ATK is just weak enough that even Pokemon like Slowbro and Suicune don't really care about it. When we're talking about a meta with threats like Landorus-T, Skarmory, and Azumarril-- damn, you're REALLY up a creek without a paddle.
As for bulk, yes Zygarde has tremendous bulk, but it doesn't have the typing or support tools to be a good wall. It only has offensive tools.
So, going offensive, the natural route is to use that bulk to set up for a sweep, and it has the tools for that (Coil and DD). Unfortunately, most of its main competitors ALSO have great set up tools, and great bulk. Garchomp is no Zygarde, but it IS still one of the statistically BULKIEST Pokemon in the tier. Garchomp's 108 / 95 / 85 makes it statistically bulker than Heatran, and on par or better than Jirachi/Celebi/etc. Zygarde's a lot bulkier yes, but they're still both going to be 1-2HKO'd by Ice and Dragon attacks. There's also Dragonite to consider. Both of these Pokemon have offensives that are a lot scarier-- that 100 ATK is just too weak.
Zygarde's only real saving grace is that this meta is so chock full of Priority, and really powerful priority... Talonflame's Brave Bird, Aegislash's Shadow Sneak, Lucario's Extremespeed/Bullet Punch, Kanghaskan and Mawhile's Suckerpunches, Azumarril's Aqua Jet, and there's still Scizor, Breloom, and Dragonite to account for as well. When it comes to these prioirty attacks, the difference between Zygarde's bulk and Garchomp's bulk is huge-- especially if we're talking about Coil. Will it be enough? I doubt it. This is a metagame where even Garchomp isn't really a stand out, and Dragon / Ground coverage has never been worse than it is now.