Well, many people have been asking my opinion about this suspect, either because I have been spamming a lot of Zygarde variations lately in tournaments (and almost always having success when doing so) or because I have been arguing in favor of a suspect in OU for some time now, so yea there I go.
Firstly I need to say that this suspect is REALLY crucial right now, since we probably will not have any other in SM OU before the new gen gets released, plus Zygarde has not an obvious attribute that makes it ban worthy. It has many checks and counters, which seems fine at first, but none of those are reliable to stop the most used Zygarde sets at the same time (barring mew, it checks the most relevant variations but just dies to +1 Z-Stab anyway) and the main reasons that makes zyg an unhealthy presence are its uncompetitiveness thanks to its ability to wear down, overwhelm and/or lure supposing checks in such an easy way (and never having big drawbacks), an incredible capacity to support many offensive threats offensively and defensively (be it via wallbreaking or simply spreading bad status) while not needing much support of team mates to do its work, huge defensive stats to a bulky-offensiveish mon and besides it all, if thats not already enough, the fact that NOTHING is immune to its main ground-type move (also stab), which is definitely stupid and unbalanced, just by itself.
Talking about its versatility: no, it DOES NOT need the right set to win games, or even to do something in a game, simply because it has all it needs almost every time since Thousand Arrows always hits, and every single set works around wearing down supposed checks to either just spam arrows at some point later in the match by spreading its own bad status and coverage, or making team mates way more dangerous thanks to its offensive support, which is a thing that no other pokemon in OU does except for Greninja (thats also suspect material, by the way). And even when it has like "the right set" to win, seems absurd to me that a mon can have the right path to run over the opp's teams in such a recurring way. Literally every single used set has enough potential to be relevant in almost every matchup, and yes, it does limit the amount of options in a team building. I cant simply use tapu bulu and pretend like I dont have problems against Zyg, or even Tangrowth and Clefable: I need ways to check it and counter it too as a backup plan, because at least two out of the 5~6 MOST USED sets (I actually meant the most used, there are like 5 extra movesets that are also good enough) abuse these pokemon.
Some other things should be mentioned, like: bulky Tapu-Bulu (specially the BU one) DOES NOT check the main zyg set (coil + bad status), the Toxic version just wins for obvious reasons (also winning if your check is Tangrowth) and Glare gives room to outstall bulu's PPs since Horn Leech does not break Substitute in one hit after the Grass Terrain's effect goes away
[calc] and Coil + Glare have such a big PP number that its really difficult to win in a scenario like this one (and im not even considering the SD + Synthesis version, whereas just loses pretty easily after getting paralyzed). SD + Ice Fang Gliscor is almost as good as mew when checking Zyg, except considering that you need to be faster to actually win the 1x1 AND can't simply throw it in the first time before checking the zyg set, otherwise you can potentially get paralyzed and just lose anyway (and will lose a considerable bulky to run so many speed too). Clefable, Latias-Mega and Landorus-T are fine checks but are way more susceptible to Zyg's natural ability to lure and wear down checks, since clef and lati hate getting paralyzed and landorus always gets chipped naturally against either zyg or standard threats during the games. I could even mention scizor but it always needs full HP to check CB arrows + it literally forces a 50/50 of who gets the crit first and zyg always has the substitute advantage + the paralyze factor on its side anyway so yeah, just another try to check broken zyg without many success.
Last but not least, the ground-type is known since the first gen as one of the best offensive types ever. Having ground immunities has always been one of the main things to consider when building a team because Earthquake gives a plethora of targets and a more than good coverage option to offensive threats, and after gen 4 earth power just added more sense to this need. Now, Thousand Arrows abuses this offensive type with mastery, ignoring ground immunities and making grass types the only answer to this move, which is not an issue to deal as seen by the zyg's coverage, typing, stats, etc.
Just to finish, here are some replays in where zyg was supposed to be checked but simply found a way to win or be relevant:
https://replay.pokemonshowdown.com/smogtours-gen7ou-404270 - OLT Vs. Zuch - Got haxed a lot but zyg just did what its supposed to do and was a pain to deal with. I had tang and landorus.
https://replay.pokemonshowdown.com/smogtours-gen7ou-400833 - OLT Vs. ABR - chipped his lando and just won with a DD version, showing how zygarde can just click setup and damage moves without drawbacks. He had clef and landorus.
https://replay.pokemonshowdown.com/smogtours-gen7ou-400834 - OLT Vs. ABR - Was really difficult to him not getting overwhelmed by zyg + ts, id prolly not click glare vs his tang if I had not a plan to possibly lure that with landorus. free turns against an already passive mon is always appreciated anyway. he had tang and offensive counters.
https://replay.pokemonshowdown.com/smogtours-gen7ou-395433 - Snake Vs. RedEmption - All I needed to do here was clicking arrows like twice vs his lando and then I had a clean sweep after a DD or two. Unfortunately I wasnt on my best day and he did what he needed abusing of a possible missplay by my part. Zyg was huge anyway, just wanted to show again that it doesnt have drawbacks in just spamming broken arrows.
https://replay.pokemonshowdown.com/smogtours-gen7ou-306812 - WCOP - John Vs. BlackOblivion - Not recent ofc, but evey time someone talks about zyg's ability to win vs anything this game comes across my mind.
I had a replay of a zyg winning vs BU bulu but I couldnt find it, I will try again tomorrow or w/e. From now I say that I will lose many teams if zyg gets banned because I really find it broken and spam as much as I can, but even considering it
I think banning zyg is the way to go and the meta will develop positively after doing it so.