I've found that Rain can be successful in Ubers. Kyogre is a million times better than Politoed and Mega Swampert just laughs in Primal Groudon's face.
...Rain only works as long as Primal Groudon is already gone. The fact that Primal Groudon is on about 80% of teams cripples rain outright. Standard Kyogre's viability absolutely plummeted when Primal Groudon was introduced; it went from S-rank in viability to B-rank. Rain
can be successful, sure, but the fact that you have to take out Primal Groudon before it can even be considered hurts its viability a lot, since Desolate Land prevents Rain from ever activating (barring Primal Kyogre's Primordial Sea, but that ends when it switches out).
Also Mega Swampert and Primal Groudon both 2HKO each other, and Primal Groudon outspeeds it so it's hardly laughing at Primal Groudon.
252+ Atk Primal Groudon Earthquake vs. 100 HP / 0 Def Mega Swampert: 211-250 (57.6 - 68.3%) -- guaranteed 2HKO
252+ Atk Primal Groudon Precipice Blades vs. 100 HP / 0 Def Mega Swampert: 255-300 (69.6 - 81.9%) -- guaranteed 2HKO
252+ Atk Mega Swampert Earthquake vs. 128 HP / 0 Def Primal Groudon: 266-314 (71.3 - 84.1%) -- guaranteed 2HKO
Also, with regards to Perish Song... it's only on 10% of Gengar in Ubers. There's far more to Mega Gengar being so dangerous than just Perish Song.