I am relatively new in this tier, and I possibly have no knowledge about it (moreover, I am from Spain, so you may don't fully understand my sentences), but I want to talk about rain abusers (or weather abusers in general). I didn't want to mention Tera in every one of my arguments in support of playing rain teams, but I have no option.
Weather setter

The only rain setter of the tier, you really don't need more. Good moveset (U-turn, Defog, Scald and Roost), allowing it to pivot, eliminate hazards (I don't think it is good for this because it is crippled by rocks, but I have always seen Defog in the movesets Pelipper uses), cripple physical attackers and cure itself. Wait, isn't Pelipper x4 weak to Electric? Well, you are forgetting it can Terastallize into Ground type and become immune to Electric-type attacks, what an incredible mechanic. Also it is quite easy to select his partners, every rain abuser you can imagine fits well with Pelipper. And here is where it gets interesting.
Rain abusers
Water types:

These Pokémon are a big headache, at least in low ladder (where I came from). Their abilities + buffed STAB attacks, make them unwieldy threats. ¿How can you stop them? Maybe some Grass-types like

, which can't switch-in against none of them due to its low defensive stats;

or

, maybe; but for me it's not that easy when these monstrous attackers have rich movesets and access to moves like Ice Punch (do I have to say it could become STAB if you Terrastalize?) or CC, among many other coverage options.
Zapdos;

Thunder and Hurricane, STAB 110 BP 100% accuracy, Weather Ball 100 BP 100% accuracy... what the hell is this? Good ability against physical sweepers, recovery... Must be in every rain team no doubt.
What are the main counters to rain? Maybe other weather setters/abusers like Charizard-Y with Solar Beam, surely no sand abusers; maybe Trick Room, but it is usable in the tier? . Do you think rain teams are powerfull in this tier? ¿Will they be strong if we ban Tera? I am looking forward to see more competitive and expert people's opinon.