Maybe I am just a bad player or I've been using Heatran wrongly but why is Heatran considered so good and splashable in OU? It's said to check a lot of things but from my experience using it (maybe I'm doing it wrong) it just doesn't do its job as well as people claimed.
1) Electrics: Most of the time, it's playing out like this. If it's SpD Heatran, it takes Rocks damage + Thunderbolt, then the Electric Volt Switches out while Heatran does whatever it wants. After that, it's in 2HKO range of Thunderbolt if it switches into Rocks again. Offensive variants simply faint to 2 Thunderbolts so it can't even switch-in.
2) Fairies: Most Fairies such as Azumarill, Mega Altaria, Mega Gardevoir and Mega Diancie carry Super Effective moves against it and with the exception of Azumarill, they all outspeed it. Even Clefable has a chance of beating it if Clef already has a CM up when Heatran switches in (unless Heatran is an Offensive variant).
3) Steels: Excadrill stomps you. Bisharp OHKOes you at +2 after Rocks. Mega Meta's Hammer Arm OHKOes you after Rocks. I guess it does win against Ferrothorn, Mega Scizor, Skarmory, Jirachi and Klefki.
4) Grass: You win against non-Earth Power Celebi, non-HP Ground Serperior and Chesnaught (Hammer Arm 2HKOes you though). Breloom stomps you. Mega Venusaur wears you down with Leech Seed while you can only fish for a burn.
5) Dragons: Garchomp stomps you. Kyurem-B Earth Powers you. Lati@s has Surf / Earthquake to nail you. You do wall Zard-X who do not carry Earthquake but you still can't do shit to it except Toxic. Offensive variants can use Earth Power against it, however.
6) Other stuff: You win against Mew and SpD Sableye, I guess. However, CM Eye uses you as set-up bait.
So, yeah, I think I've listed things people claimed it can do but didn't seem to work as well in practice.