I'm not very high up on the ladder, but I threw Zygarde on my team to see whether or not it could work for me and how well. I've been using the SubCoil set, and while its good, I don't think it's broken in the sense that you can just slap it on a team and make it work. While it's quite bulky and has great typing, its low damage output before setup means that it can be stopped fairly easily by faster threats that can status it, so cleric support is pretty much necessary. It also needs entry hazard support to function well, otherwise it just won't nail some KOs that it really needs to. Unlike other Pokemon that were deemed broken, it does have a number of good counters and checks to it. Some people here mentioned Whimsicott, but pretty much any faster Pokemon which can catch Zygarde on the switch and status it works well. Maybe it's my lack of skill that makes me unable to use Zygarde at full potential, but I just don't think that it's broken.