Heatran is a very reliable pokemon, but he does have several hard counters. Salamence is near impossible to switch into, because you have no way of determining its set (ie Mixed or DD or even Bulky DD) and even if you do you have to successfully predict its opening move. (Sending Jirachi into Fire Blast or Starmie into Draco Meteor sucks). On the other hand, Heatran is predominantly a special attacker so Vaporeon, Suicune, Tyranitar and the like all can switch in relatively safely. You don't find Heatran opening with HP Electric/Grass or Explosion most of the time.. Heatran has to rely on Explosion to muscle through "hard counters" such as Suicune and Porygon, which makes it much easier to handle/predict around.