Are people fucking retarded? or just /vp/ being shit in comp?
Zygarde is Bulky as fuck. if you can use 100% (hell, even 50%, even 10% is quite good) it will make it better than Garchomp and cuck Flygon once more because -
Getting Cells and cores allow you to teach him Thousand Arrows/waves, Dragon Dance and EXspeed.
Outrage
Exspeed
DragonDance
Thousand Arrows
Guess what? you have almost EQ that hit ANYTHING. Zygarde is bulky as fuck, so it can fucking DDance for sure. Literally the only thing that you can't hit for regular damage is Grass/Fairy or Bug/Fairy, which you can hit with Stone Edge/EX speed
Hell, even if you can't use 100% Zygarde, after getting into lower than 50% (which is possible because of it's bulk), you recover HP and have even more bulk.
Zygarde is going to be OU at least. Thousand Arrows is pretty much the one thing he needed the most in gen 6 to get out from BL.