Since a lot of people keep posting about Zygarde and I was one of the main people pushing for it to stay A+, I'll offer my reasoning as to why I believe that.
While Zygarde is extremely splashable, it is simply not as good defensively or offensively as the other S rank mons. It isn't that scary of a breaker, and it certainly isn't hard to revenge kill. I don't think I need to tell anyone why CB is pretty easy to check, but I'll briefly list the reliable switchins to CB (mostly Thousand Arrows cuz outrage is hardly being clicked): Tangrowth, M-Bro, Defensive Lando (lefties preferably), M-Sciz, Bulu, M-Hera, Clefable, Mew, Alo, etc. CB isn't threatening enough by itself to warrant S, and it seems that most people are acknowledging this, but still see other sets like DD as making it S-worthy.
The thing with DD, however, is that it's still mostly checked by the same things defensively, and still easy to revenge. While Zygarde may end up killing something like Tangrowth at +2, this is a one time thing and it is still weakened in the process (like from hpice). So the tradeoff will involve something like using up a Z move once and ending up at 40%, only to not be threatening at all the remainder of the game. M-Bro will still fully beat it; Defensive Lando either beats it or does something similar to Tangrowth (but intims also), M-Sciz can get like 4 bullet punches off if it comes in on DD lol, Bulu obviously still beats it, both Clef sets beat it, Mew takes +1 drag-z and wisps/ibeams, Alo still toxics, and so on. Almost all of the checks to CB and DD overlap, so I don't get why you'd use the argument that scouting it's set is an impossible and important task - it's neither.
As for revengers themselves, there are the plentiful scarfers such as Latios, Greninja, Garchomp, and Lele that all revenge Zygarde with ease. And for CB sets, many pokemon like Zapdos also revenge very easily. Considering this, in addition to the aforementioned defensive counterplay, Zygarde just is not as threatening as people are making it out to be.
Now, if Zygarde is looked at in direct comparison to Lando/Magearna, it becomes even more evident why it shouldn't be S. Defensively, Landorus and Magearna are leagues better than Zygarde. Lando is a good check/counter to: Zard-X, Garchomp, Heracross, Marowak, Mawile, Mimikyu, Bulu, Tyranitar, Zygarde, and more. As for Magearna, the defensive utility of AV is pretty insane. It reasonably switches into: Zam, Clef, Gengar, both Grens, Hoopa, Kyurem, Latios, OTR/shift gearna, Mane, Nihilego, Lele, Koko, and so on. It even can volt on magma trans and take less than 45. Offensively speaking about these pokemon, there are probably 0 surefire counters to Lando besides M-Bro, Quagsire, and Skarmory (can still lose to smackdown sets). And for Magearna, both shift/otr are very powerful sets with about 3-4 possible z-moves, all hitting different targets. Mew is safe vs boltbeam fight-z, but loses to z-fleur. Heatrans safe vs the fleur set, but loses to fight-z. Celes is reasonable vs both of those, but fears elec-z. If anything requires multiple defensive answers and set scouting, it's Lando and Magearna, not Zygarde.
All in all, this pokemon just isn't on the same wavelength of potency as the current S rank mons.