I was very unsure whether to put Zygarde above or below Eternatus, but ultimately settled for below it on the basis of it being something I actually struggle to find a place for on teams.  While teams built around it are good, often on teams that aren't, there's something else that will provide more value or check something that Zygarde doesn't.  The big thing I find myself constantly worrying about in teams is the Primal Kyogre matchup, which Zygarde doesn't help.  I'm not entirely sure how to put it into words, but Zygarde isn't as splashable as it looks, to me at least.