Yes, they're viable. There are definitely better revenge killers available, but Heatran has unique coverage that a Scarf can help it use against more Pokemon. It functions as a decent late game cleaner as well as a Fairy check.
It's main perk is how unexpected it is. Most Heatran use bulky attacking sets as a way to counter numerous threats such as Talonflame and the Lati duo, so this is the set that many player will expect. Scarf Heatran can then surprise the opponent with its unexpected speed that allows it to revenge kill numerous Pokemon, particularly Fairies who die to Flash Cannon. While regular Heatran sets do a good enough job against Fairies like Sylveon and Clefable, it dies to faster Fairies like Mega Gardevoir (who has seen a rise in usage) that pack Focus Blast. Scarf Heatran is a way to beat those Fairies who want to drop a Focus Blast on Heatran.
Choice Scarf Heatran does have problems though, including:
- Obvioudly it get locked into a move, which forces it to switch quite often.
- Once the opponent finds out it's Scarfed, it becomes extremely predictable. If they can anticipate which attacks you're going to use, the consequences are huge, as there are many Pokemon that find Heatran's Choice-locked moves as a free switchin.
- It's slow for a Scarf Pokemon and will not outspeed most Pokemon that are at +1, so do not use it as a way to eat Speed boosting Pokemon.
- It can't have an Air Balloon. Although this may seem minor, the Air Balloon allows so many switchin opportunities against Pokemon like Garchomp and Excadrill, who can't touch Heatran without their Ground attacks.
- Other things I'm forgetting.
Don't care about the set so I'll steal one from Smogon.
Heatran @ Choice Scarf
Ability: Flash Fire
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature (or something that raises Speed and lowers a defensive stat)
- Flash Cannon
- Earth Power / HP Ice
- Stone Edge
Overheat hits as hard as possible. SAtk drop matters little as the opponent's switchin will most likely resist Fire attacks. If late game sweeps are your thing, Fire Blast and Flamethrower are decent alternatives. Flash Cannon beats Fairies. Earth Power hits Fire types and opposing Heatran who wall you, but HP Ice lets you hit x4 weak Dragons before they hit you with Earthquake. Stone Edge hits Volcarona and Charizard Y.