[Dark Fallen Angel]

But this is only true if you are using a bulky version of Rotom-H; and I think that, at least on sun teams, not using Rotom-H with Choice Specs is a waste of its wallbreaking potential. It may be not, but in my opinion the best way to use Rotom-H in sun is with Choice Specs, as that makes its Overheat extremely powerful. Here are some calculations:

(Even the pink blobs take a hefty amount of damage)
252SpAtk Choice Specs Rotom (Neutral) Overheat in Sun vs 252HP/4SpDef Eviolite Chansey (Neutral): 40% - 47% (282 - 333 HP). Guaranteed 3HKO.
252SpAtk Choice Specs Rotom (Neutral) Overheat in Sun vs 4HP/252SpDef Leftovers Blissey (+SpDef): 39% - 46% (258 - 304 HP). Guaranteed 3HKO.

(As Quagsire's Unaware ignores the SpA drop from Overheat, even the most specially defensive variants of Quagsire may as well say bye bye)
252SpAtk Choice Specs Rotom (Neutral) Overheat in Sun vs 252HP/252SpDef Leftovers Unaware Quagsire (+SpDef): 52% - 62% (207 - 245 HP). Guaranteed 2HKO.

(Grim Reaper has a thing to say to any Porygon2 that doesn't invest in Special Defense)
252SpAtk Choice Specs Rotom (Neutral) Overheat in Sun vs 252HP/252SpDef Eviolite Porygon2 (+SpDef): 58% - 69% (220 - 259 HP). Guaranteed 2HKO.

(Assuming that Vaporeon switches correctly expecting Overheat and doesn't have investiment in SpD, as most Vaporeon are physically defensive)
252SpAtk Choice Specs Rotom (Neutral) Overheat in Sun vs 252HP/4SpDef Leftovers Water Absorb Vaporeon (Neutral): 49% - 58% (229 - 270 HP). Guaranteed 3HKO. 16% chance to 2HKO with Leftovers.

(The most specially defensive version of Ferrothorn is not going to survive Specs Rotom-H's Overheat anytime soon, even in rain)
252SpAtk Choice Specs Rotom (Neutral) Overheat in Rain vs 252HP/252SpDef Leftovers Ferrothorn (+SpDef): 109% - 129% (384 - 456 HP). Guaranteed OHKO.

(Not even Slowbro, Tentacruel and Gastrodon escapes from the sheer power of Overheat)
252SpAtk Choice Specs Rotom (Neutral) Overheat in Sun vs 252HP/4SpDef Leftovers Slowbro (Neutral): 67% - 79% (264 - 312 HP). Guaranteed 2HKO.
252SpAtk Choice Specs Rotom (Neutral) Overheat in Sun vs 252HP/0SpDef Leftovers Tentacruel (Neutral): 51% - 60% (189 - 222 HP). Guaranteed 2HKO.
252SpAtk Choice Specs Rotom (Neutral) Overheat in Sun vs 252HP/252SpDef Leftovers Storm Drain Gastrodon (+SpDef): 42% - 50% (180 - 213 HP). Guaranteed 3HKO.

(Hippowdon must think twice about switching in even with max investiment in Special Defense, and this is in Sandstorm!)
252SpAtk Choice Specs Rotom (Neutral) Overheat in Sandstorm vs 252HP/252SpDef Leftovers Hippowdon (Neutral): 68% - 80% (288 - 339 HP). Guaranteed 2HKO.

(Terrakion and Kingdra are not walls, but I think that they are worthy of mention, Terrakion because it's rather bulky and resists Overheat, and Kingdra because it 4x resists Overheat)
252SpAtk Choice Specs Rotom (Neutral) Overheat in Sun vs 4HP/0SpDef Leftovers Terrakion (Neutral): 74% - 87% (242 - 285 HP). Guaranteed 2HKO.
252SpAtk Choice Specs Rotom (Neutral) Overheat in Sun vs 4HP/0SpDef Leftovers Damp Kingdra (Neutral): 39% - 46% (115 - 136 HP). Guaranteed 3HKO.

And this is not even considering that Slowbro, Vaporeon and Tentacruel are weak to Volt Switch/Thunderbolt!

So that's it; I'm not using Rotom-H just to have a counter to Genesect, and alas, sun teams fare well against Genesect. In my opinion the best way to use Rotom-H is, and always will be, as a wallbreaker. And these versions of Rotom-H do not survive +2 Sludge Bomb from Venusaur at all.

So if you manage to bring Rotom-H safely and the opposing Pokémon can't defeat him, you are sure that even the next switch-in is going to take a large amount of damage, if it is not outright OHKOed.