I think part of the reason why Tangrowth dropped is because of Kyurem-B and U-turn spam from the like of Pheromosa & Genesect. Slowbro suffers from the same problem but it has access to Teleport to keep momentum, while Buzzwole or Tangrowth just kinda sits there. And with the meta is so incredibly volatile, these mons doesn't have a minute to breathe in OU right now.Considering that Zygarde is such a problem, it's kind of weird that both Buzzwole and Tangrowth dropped into UU considering that they're probably the best defensive answers to it. Buzzwole I understand that it's a very niche pick, but Tangrowth especially weirds me out.
About Zygarde, every time it has a Substitute up, my heart sinks. If I switch to get a check in then I get a Glare in the face, if I don't switch then it just proceeds to do its Dragon Dance/Coil nonsense. Zygarde doesn't even need to set up to win, it could just rinse & repeat the whole the Sub/Glare process to cripple your offensive mons and let its teammate grab the win instead. Right now Zygarde is the thing that you know exactly what it is about to do (aside from the occasionally CB set), but trying to stop it hurts.