What makes Chandelure and Obstagoon so good in today's SS OU and Hatterene worse in SS OU nowadays?
Chandelure is a very strong Choice Specs wallbreaker that can come in on the standard Wish + Teleport Clefable, doesn't have a lot of switch-ins, and can cripple passive Pokemon with Trick.
Obstagoon is a great balance breaker: it doesn't have to care about status (after being burnt), threatens a lot of the current metagame with Facade and Knock Off (and Close Combat), has good utility in Knock Off and Switcheroo, setup potential with Bulk Up, and good natural bulk to boot.
Additionally, wallbreakers like Chandelure and Obstagoon enjoy Clefable being able to bring them in safely with Teleport, which is as common as ever.
I think Hatterene falling off is in part due to people being prepared for Clefable. Most teams have threats like Excadrill, Aegislash or Jirachi that can be pivoted into.
What's the reasoning behind Celebi's drop in the VR? It's defensive Colbur set being able to switch in on and beat Vish, Conk & Zeraora seems like it's quite valuable, I understand it's not the most reliable and can give up momentum, especially as it's u-turn bait and has to recover more than it'd like, but it has a decent amount of utility with absorbing status, T Wave, rocks (though it's a bad setter since it lets Corv in) or Earth Power to keep most steels out and it can always u-turn itself out as well. At the very least it feels like it can perform this role of checking these A and A+ threats on many teams relatively reliably, especially on teams foregoing Toad for Kommo or other water types that can't check Vish as reliably.
For one, this Celebi set can only check Conkeldurr and Zeraora once as long as it keeps its Colbur Berry, which makes it quite unreliable. Additionally, outside of checking these specific threats, its Grass/Psychic typing is quite terrible: most of its sets can't do anything to Corviknight, which is still definitely a top 3 Pokemon, and it has some crippling weaknesses that hold it back from providing reliable defensive utility. Finally, it has close to no solid tournament results off the top of my head, which is quite telling and is often taken into account when working on the VR.
Whats the reason for salazzles bizarre 5 rank rise in vr? What does it even run?
Its ability to poison virtually everything with Toxic, including Toxapex and Steel-types, can prove to be exceptionally valuable in this balance-dominated metagame. Clefable obviously isn't a good switch-in to a Poison/Fire Pokemon either. It always runs Toxic, usually either with Heavy-Duty Boots and 3 of Sludge Wave/Venoshock, Flamethrower, Knock Off, and Nasty Plot or with Black Sludge and Substitute + Protect + Sludge Wave/Venoshock/Flamethrower.