Rain isn't really viable in Gen 9 PU
I asked on Smogon, and the responses I got (you can read the full responses here and here.) said Rain isn't really a viable archetype to use in PU. Here are a couple reasons why:
Swift Swim users and Rain Setters: There aren't any Drizzle users, so you're forced to use Manual Rain, which isn't as consistent as Auto Rain. You also have very few options for Rain abusers, being Poliwrath, Floatzel, and Hisuian Qwilfish. However, the rain abusers, specifcally Poliwrath and Floatzel, do fine outside of Rain, so they don't need rain to have success.
(Grassy) Terrain is better as an HO archetype in PU: Terrain teams in PU are more consistent as HO archetypes in PU, specifically Grassy Terrain. Grassy Terrain has quite a bit of abusers to make it more versatile. Grassy Terrain has Unburden users such as Sceptile, and Grafaiai for speed control, as well as having some Grassy Glide users, making it a faster HO archetype than Rain.
TL;DR Rain isn't really viable in PU due to there being few abusers, having to use Manual weather setting, and Terrain teams outclassing rain teams as a HO archetype.
