Yea, 100 speed is pretty mediocre, it needs awkward setup and it doesn't have an item slot. Getting it down to 50% HP and surviving another turn will be really awkward as well, considering a lot of things will be able to set up or OHKO it since it doesn't start with the 212 HP. It'll also be practically dead weight pre-Mega, as it'll be attacking with fairly weak special moves and hard walled by a few things. Hoopa-U was barely even OU this gen and that thing has 160/170 mixed attack stats, way more variety as well as the ability to hold an item to bring its effective attack far above Mega Zygarde. It does obviously have a much worse typing and narrowly worse speed, but big numbers aren't everything.
Complete Zygarde is pretty scary without mega in fairness, but a lot of that comes from having a ground move which can hit flying types, and it's inconsistent because it needs to get down to half before it's any good.
If it does get sent to AG (which is still very possible), it will most likely be because that 0 immunity dragon move is braindead and forces very specific checks.