Ok so now that Ive talked about some of Zygarde's great things, I'm gonna bring up some things that make me want to say keep it in UU.
(Disclaimer: I will be talking about the SubCoil set, as it is much more popular and prevalent.)
1. Zygarde is easily worn down by Toxic Spikes
Zygarde's most important attribute is it's longevity. This is common knowledge. Zygarde's entire job is to get up a substitute, and basically stay behind it, gathering leftovers recovery, and forcing opposing mons out, or dealing damage to them. So, what cuts down nearly any pokemon's longevity severely? Well, Toxic Spikes do. Toxic Spikes put Zygarde in a position where he can't risk his own HP on a substitute, and he simply put, cant do what he is meant to do. This means that Zygarde needs support from an extremely reliable Spinner, Defogger, or grounded Poison-Type to help out in clearing the spikes. Spinners are Generally not as longevitous as Zygarde is, and the outcome will generally be the Spinner going down long before Zygarde. Defoggers are primarily Flying-Type in UU (Crobat, Aerodactyl, etc.), and stack an Ice-Type weakness with Zygarde, meaning this will then take some teambuilding support to alleviate the Ice Weakness. Grounded Poison-Types are scarce enough in UU, leaving the only viable one at the moment, Roserade, who stacks Ice Weakness with Zygarde as well.
2. Zygarde has no recovery aside from Leftovers
Probably Zygarde's biggest downfall as a longevitous Titan is it's lack of reliable recovery outside of Leftovers. This means that Zygarde relies heavily on his item to recover the HP he took to make a substitute. If his leftovers are knocked off, this results in Zygarde becoming much less of a threat, as avoiding status with Substitute comes at a much greater cost.
3. Zygarde's checks are healthy for the metagame
Yeah, I know it sounds ridiculous. But Zygarde's checks are things that had purpose on teams before, but must be tweaked slightly to adapt to the ever changing metagame. Stale Meta is no fun, everybody knows this. So with Zygarde here, it puts a nice twist on the meta, leaving some pokemon more useful than they were before, and meaning some sets had to be changed to still be viable in UU. Some notable cases of this include: Crobat running Haze/Infiltrator Toxic, Granbull's jump in viability, Whimsicott becoming a powerhouse, Encore Mega-Alakazam, Stall's decline in viability, Hyper Offense's raise in viability, and many others. Zygarde leaves the UU meta in a strange state at the moment, but when everything settles, The meta will be shaken up in a way such that we get a new era, per se, of UU. Seeing such a shift of successful playstyles gives me hope that Zygarde will usher in a new metagame in which diversity is healthy.
Until then I can only hope.