My favorite antagonistic team. I don't care about their brief and somewhat goofy implementation in the postgame story. I just love how Kazumasa Iwao and friends found some way to justify BEING COMPLETELY UNAFRAID TO GIVE NOTABLE TRAINERS LEGENDARY MASCOT POKÉMON FOR ONCE IN A MAINLINE GAME by having a Giovanni come from some world he was more successful in and piecing together an entire team of more successful villainous leaders with their own legendary mascot Pokémon in attempt to take over the multiverse. Just this group alone had a bunch of villains with more developed teams than any other villainous team combined, because for the longest time, the bar was considerably low. Trainers who pose as region/world-level threats should have the Pokémon to back them up. Trainers said to be some of the most powerful in the series should have the Pokémon to back them up. Powerful legendaries, mythicals, etc should be used and fought off against more. This was a childhood dream come true, and fighting them off wasn't even too much of a challenge.
Yeah the player did quickly blow through their entire organization and defeated them but in all fairness this was after they had to deal with Ultra Necrozma by themselves with their Pokémon. We were qualified to easily handle this. It's fine that they weren't the primary focus of the story because there wasn't anything much they could do without it being repetitive filler. USUM's story was also largely about the story of Alola, Necrozma's influence on the region, aftermath regarding Necrozma's influence, and people forming different responses to find their own ways of dealing with said aftermath. There wasn't much room for Team Rainbow Rocket to be the bulk of this story and it had little to do with the world of Alola itself.
I don't care about this plotline being an attempt to pander to old fans because it was done in a way that truly gave something the mainline games should've had for a while and I don't get the complaints as this was significantly less panderous than the so-beloved PWT, which featured almost exclusively gym leaders and Champions from the first 5 gens and had its main appeal over normal battle facilities just be that...
(If you disagree with this last point, be honest, would you have enjoyed/praised the PWT if the only trainers were all random NPCs instead? I would be praising Team Rainbow Rocket even if they brought up completely new trainers instead of old villains simply due to the Pokémon they brought for us to battle off against and the stakes fitting accordingly to such. It was refreshing for a core series game to finally do this.)