I think the rise of Banded Zygarde is interesting, but also logical. It's somewhat similar to Banded Dragonite from last gen, only...better in a lot of ways?
1. General Bulk/Typing. Sure, not having Multiscale sucks especially because Zygarde's ability is literally useless in OU, but Rocks and chip damage meant Dragonite couldn't abuse Multiscale as often as it would like (compared to say, a DD/Roost set). In addition, Dragonite is weak to Stealth Rock, really hindering the lifespan and ability to switch in repeatedly, and Zygarde resists Stealth Rock (while taking full damage from Spikes and Toxic Spikes isn't great, it's arguably better than being weak to Stealth Rock). Meanwhile, while Dragonite's 91/95/100 bulk is quite nice, but 108/121/95 bulk is substantially more (especially on the physical side), and not having a weakness to SR is huge.
Dragonite is also hindered by all four terrains, while Zygarde benefits from them: Burn and Toxic are great ways to either neuter Band Zygarde's power, or put it on a timer and decrease the effective bulk it has, and Misty Terran negates that (Dragonite can still get Burned or Toxic'd, both break Multiscale and either cripple BandNite or put it on a timer); Psychic Terrain is less useful because it stops your Extremespeed...but it also stops Ice Shard from Weavile and Mamoswine which can pick you off easily (Dragonite still gets picked off by Ice Shard, while not being able to use Extremespeed at all); Grassy Terrain gives you extra recovery and halves the power of Earthquakes thrown by Landorus-T (most often) at you, while not hindering the damage your Thousand Arrows does (Dragonite doesn't get the extra recovery while halving the power of the most common coverage move it uses on the Band set, Earthquake); Electric Terrain....well, Zygarde is immune to Electric attacks anyways and I guess you can't get put to sleep, but at least you're not hindered by it (Dragonite does not enjoy taking +1 Electric attacks in the slightest).
Zygarde is also blessed with an immunity to Electric moves, something very useful in a meta with stuff like Tapu Koko running around, while Dragonite can be Volt Switched on for momentum (obviously U-Turn still works and Koko runs that more often now, but still, Thunderbolt from Koko/Magearna/Lele are reasonably common). This all means Zygarde generally can switch into more stuff (better overall bulk) more often (no SR weakness).
2. Thousand Arrows. This is probably the biggest thing. BandNite hits noticeably harder than Zygarde, but it also could not make use of its secondary STAB, and while it had good coverage (Elemental Punches, EQ) and good priority (Espeed), not being able to use secondary STAB is never ideal. In addition, Bandnite was commonly walled depending on what it was locked into (Elemental Punches walled usually by Tran or Ferro, EQ by anything with Levitate or Flying, Outrage by Fairies, Extremespeed by anything bulky, TOGEKISS). Meanwhile, Zygarde can spam Choice Band boosted Thousand Arrows and hit everything, even if it's a resist. Stuff like Skarmory or the Lati twins or Landorus-T take valuable chip damage each time they come in on Thousand Arrows, and you don't actually lose much momentum if they come in (relative to EQ/Dragon spam, which things are actually immune to).
This, coupled with Zygarde's great bulk, lets it sit there as stuff like Skarm and Lando-T come in to check/counter you and get chip damage as they phase you out or something, instead of just losing momentum/being set up for things like Mega Charizard (both forms), etc.
